deriv LSK ETT STT aSTA ALPH OLDHOMEPAGE NEWHOMEPAGE
"ftsoss" means "For The Sake of Subsequent Sutras".
Example. sUtra turiSThemey... mentions "tR तृ before /iSThan /iman /Iyasun", but this sUtra would have worked fine if it were "tR तृ before /iSThan /Iyasun", because tR तृ never appears before /iman. The /iman is FTSOSS — it is mentioned in this sUtra only so that next sUtras inherit it.
Of course, this sUtra might have had "before /iSThan /Iyasun" and the next sUtra might have had "and before /iman too". That would have been simpler — but longer. Instead of
tur iSThemeyassu | TeH | तुरिष्ठेमेयःसु । टेः । it would be tur iSThemanoH | Ter iyasi ca | तुरिष्ठेमनोः । टेरियसि च ।. See pANini's razor.