The way that <st> works is that rather than target the player then hit your ability, you would hit the ability then <st> would let you select the target. <st> also used to be referred to as "shifting target" in FFXI

Example:

/ac "cover" <stpt> (shifting target within party)

cursor would only let you cycle/tab between party members (and would ignore any alliance members). You then click a second time and the ability would activate on the target.

Its not vital, but it is a nice QoL thing, especially for those who played FFXI

This could help controller players in particular is in things such as targetting issues (trying to tab between so many targets gets frustrating to cycle through when the controler cycles through both PC and NPC players at the same time)