21 C.F.R. § 808.3
Compelling local conditions includes any factors, considerations, or circumstances prevailing in, or characteristic of, the geographic area or population of the State or political subdivision that justify exemption from preemption.
More stringent refers to a requirement of greater restrictiveness or one that is expected to afford to those who may be exposed to a risk of injury from a device a higher degree of protection than is afforded by a requirement applicable to the device under the Federal Food, Drug, and Cosmetic Act.
Political subdivision or locality means any lawfully established local governmental unit within a State which unit has the authority to establish or continue in effect any requirement having the force and effect of law with respect to a device intended for human use.
State means any State or Territory of the United States, including but not limited to, the District of Columbia and the Commonwealth of Puerto Rico.
Substantially identical to refers to the fact that a State or local requirement does not significantly differ in effect from a Federal requirement.
21 C.F.R. §808.3