The only place to find out what is in the F-Pack is the vendor. My guess is they'll not share the info, either because it's a business secret OR they really don't want you to know.
There's nothing bad in the F-Pack -- tasting it won't hurt you. It may not taste good in a concentrated format. For White Zin and Piesporter, Rocky may be correct that it's little more than sugar syrup. For other F-Packs, there is definitely flavoring.
Winexpert's Luna Bianca is a slightly sweet, heavy body chardonnay. My best guess is the F-Pack contains a chardonnay concentrate along with oak flavoring and possibly glycerin.
For the "fun wine" kits I've made, my guess is fruit juice and glycerin, and possibly extra fruit flavoring.
The port-style kits? Certainly juice and flavoring, probably glycerin. I did a "sherry" kit over 10 years ago -- I'm since wondered about the F-Pack ....
Some folks add some or all of the F-Pack to the fermenter when starting the batch. If I do another fun wine kit, I will probably add half. Most are too sweet for my taste, but bone dry would not be pleasing.