According to this site, the standard Singmaster notation you are using (with F, B, U, D, L, R for Front, Back, Up, Down, Left, and Right) has the following way of indicating middle slices:

M (Middle): the slice between L and R, as seen from L.

 

E (Equator): the slice between U and D, as seen from D.

 

S (Standing): the slice between F and B, as seen from F.

The page also notes that l, d, and f (lowercase) are sometimes used for M, E, and S, and sometimes for other things - basically it looks like you should always provide a key for any notation beyond the basic 6 letters in case conventions differ.

According to this site, the standard Singmaster notation you are using (with F, B, U, D, L, R for Front, Back, Up, Down, Left, and Right) has the following way of indicating middle slices:

M (Middle): the slice between L and R, as seen from L.

E (Equator): the slice between U and D, as seen from D.

S (Standing): the slice between F and B, as seen from F.

The page also notes that l, d, and f (lowercase) are sometimes used for M, E, and S, and sometimes for other things - basically it looks like you should always provide a key for any notation beyond the basic 6 letters in case conventions differ.