Question to Powerpass

I only have one character right now and so far I have the vern and feiton powerpass.
My question is why and when should I use them? And are they like a one time use? If i use the feiton powerpass then I’ll be at the post-feiton content where as for the vern one I’ll end up at post-vern content, no?

Well, the Feiton powerpass expires June 30, so use it before then. The Vern one does not expire.

The Feiton powerpass completes the storyline through Feiton, and awards you level 960 gear for the character you use it on.
The Vern powerpass completes the storyline through Vern and Shushire, and awards you tier 1 (level 302) gear.

As for what to use them on, use them on any alt class you want to play that sounds interesting to you. And yes, they are one time use, so if you decide you don’t like the class, deleting it will not give you the powerpass back.