When used on a prescription, "SL" refers to a specific:

In standard prescription sig abbreviations, “SL” means sublingual, indicating the route of administration (placing the medication under the tongue). It does not refer to package size, strength, or a disease state.

References (Pharmacy Technician documents / Study Guides):

• Common prescription abbreviations/sig codes used in pharmacy practice (e.g., SL = sublingual).

• PTCB/ExCPT study guides: interpretation of prescription directions and routes of administration.