The letters indicate the length of the articulation.
'L' (Long) or 'S' (Short) for the 1st Articulation
'LL' indicates the 1st and 2nd Articulation
'|' indicates the 1st and 3rd Articulation
'2LL' indicates the 1st and 2nd Articulations are lengthened samples
for example:
p-ff-p = (L|L) = Long p to |ff| to Long p
p-ff-p = (LL) = Long p to Long ff to p
p-ff-p = (2LL) = Long lenthened p to Long lenthened ff to p
p-ff = (L) = Long p to ff
https://vi-control.net/community/threads/berlin-strings-what-does-sus-imm-mean.54187/