These mappings insert font descriptions such as: \textsf{<++>}<++> with the cursor left in place of the first placeholder (the <++> characters).

Mnemonic:

Example: Typing FEM in insert-mode expands to \emph{<++>}<++>.

Just like environment mappings, you can visually select an area and press `sf to have it enclosed in: \textsf{word} or

{\sffamily
line
}

depending on character-wise or line-wise selection.