The authority for temporary tags is provided by:

SB 1134 (Passed by 2007 Legislature):

320.96  Secure print-on-demand electronic temporary license plate system.  The Department of Highway Safety and Motor Vehicles shall implement a secure print-on-demand electronic temporary license plate registration, record retention, and issue system for use by every department-authorized issuer of temporary license plates by the end of the 2007-2008 fiscal year. Secure print-on-demand for this purpose means validating state registration data using higher levels of commercially accepted data encryption methods from the point of department connectivity to the license plate printer. The temporary license plate media used for this purpose shall be an on permeable material that maintains its structural integrity, including graphic and data adhesion, in all weather conditions after being placed on a vehicle. For public safety in general and for the safety of law enforcement officers, placement of temporary license plates on the outside of the vehicle and in the provided license plate mount when available is encouraged. The department may adopt rules as necessary to implement this program in the 2007-2008 fiscal year. The department may provide such exemptions as may be feasibly required.

s. 320.131, F.S. Temporary tags.--
(1)  The department is authorized and empowered to design, issue, and regulate the use of temporary tags to be designated "temporary tags" for use in the following cases:
(a)  Where a dealer license plate may not be lawfully used.
(b)  For a casual or private sale, including the sale of a marine boat trailer by a marine boat trailer dealer. A "casual or private sale" means any sale other than that by a licensed dealer.
(c)  For certified common carriers or driveaway companies who transport motor vehicles, mobile homes, or recreational vehicles from one place to another for persons other than themselves.
(d)  For banks, credit unions, and other financial institutions which are not required to be licensed under the provisions of s. 320.27, s. 320.77, or s. 320.771, but need temporary tags for the purpose of demonstrating repossessions for sale.
(e)  Where a motor vehicle is sold in this state to a resident of another state for registration therein and the motor vehicle is not required to be registered under the provisions of s. 320.38.
(f)  Where a motor vehicle is required to be weighed or emission tested prior to registration or have a vehicle identification number verified. A temporary tag issued for any of these purposes shall be valid for 10 days.
(g)  Where an out-of-state resident, subject to registration in this state, must secure ownership documentation from the home state.
(h)  For a rental car company which possesses a motor vehicle dealer license and which may use temporary tags on vehicles offered for lease by such company in accordance with the provisions of rules established by the department. However, the original issuance date of a temporary tag shall be the date which determines the applicable license plate fee.
(i)  In the resolution of a consumer complaint where there is a need to issue more than two temporary tags, the department may do so.
(j)  While a personalized prestige or specialty license plate is being manufactured for use upon the motor vehicle. A temporary tag issued for this purpose shall be valid for 90 days.
(k)  In any case where a permanent license plate cannot legally be issued to an applicant and a temporary license plate is not specifically authorized under the provisions of this section, the department shall have the discretion to issue or authorize agents or Florida licensed dealers to issue temporary license plates to applicants demonstrating a need for such temporary use.
(l)  For use by licensed dealers to transport motor vehicles and recreational vehicles from the dealer's licensed location to an off-premise sales location and return. Temporary tags used for such purposes shall be issued to the licensed dealer who owns the vehicles.

Further, the department is authorized to disallow the purchase of temporary tags by licensed dealers, common carriers, or financial institutions in those cases where abuse has occurred.
(2)  The department is authorized to sell temporary tags, in addition to those listed above, to their agents and where need is demonstrated by a consumer complainant. The fee shall be $2 each. One dollar from each tag sold shall be deposited into the Brain and Spinal Cord Injury Rehabilitation Trust Fund, with the remaining proceeds being deposited into the Highway Safety Operating Trust Fund. Agents of the department shall sell temporary tags for $2 each and shall charge the service charge authorized by s. 320.04 per transaction, regardless of the quantity sold. Requests for purchase of temporary tags to the department or its agents shall be made, where applicable, on letterhead stationery and notarized. Except as specifically provided otherwise, a temporary tag shall be valid for 30 days, and no more than two shall be issued to the same person for the same vehicle.
(3)  Any person or corporation who unlawfully issues or uses a temporary tag or violates this section or any rule adopted by the department to implement this section is guilty of a misdemeanor of the second degree punishable as provided in s. 775.082 or s. 775.083 in addition to other administrative action by the department, except that using a temporary tag that has been expired for a period of 7 days or less is a non criminal infraction, and is a nonmoving violation punishable as provided for in chapter 318.
(4)  Temporary tags shall be conspicuously displayed in the rear license plate bracket or attached to the inside of the rear window in an upright position so as to be clearly visible from the rear of the vehicle. On vehicles requiring front display of license plates, temporary tags shall be displayed on the front of the vehicle in the location where the metal license plate would normally be displayed.
(5)  Any person who knowingly and willfully abuses or misuses temporary tag issuance to avoid registering a vehicle requiring registration pursuant to this chapter or chapter 319 commits a misdemeanor of the first degree, punishable as provided in s. 775.082 or s. 775.083.
(6)  Any person who knowingly and willfully issues a temporary tag or causes another to issue a temporary tag to a fictitious person or entity to avoid disclosure of the true owner of a vehicle commits a felony of the third degree, punishable as provided in s. 775.082 or s. 775.083, or s. 775.084.
(7)  Any person authorized by this section to purchase and issue a temporary tag shall maintain records as required by this chapter or departmental rules, and such records shall be open to inspection by the department or its agents during reasonable business hours. Any person who knowingly and willfully fails to comply with this subsection commits a misdemeanor of the second degree, punishable as provided in s. 775.082 or s. 775.083.
(8)  The department may administer an electronic system for licensed motor vehicle dealers to use in issuing temporary license plates. Upon issuing a temporary license plate, the dealer shall access the electronic system and enter the appropriate vehicle and owner information within the timeframe specified by department rule. If a dealer fails to comply with the department's requirements for issuing temporary license plates using the electronic system, the department may deny, suspend, or revoke a license under s. 320.27(9)(b)16. upon proof that the licensee has failed to comply with the department's requirements. The department may adopt rules to administer this section.