sailingfun

The S2's are not positive space. As mentioned you pay a 50 dollar per year fee to non rev travel. That covers all the passes for your family and parents. If you fly in a market with customs, immigration or tax fees those will be in addition to the one time yearly charges. The UK is the worst in that regard.
Boarding priority is by standby class. The S means standby. The number after is your class. As a employee you and your family normally ride as a S3. Parents, retirees and non dependent children are S3b's. You get 6 flying days a year where you can fly as a S2. They are informally called vacation passes. It simply moves you ahead of the hoard of S3's.
S4's are the eight passes you can give to anyone. There is a per mile charge. That cost can sometimes be the same as a discounted real ticket on domestic flights. They are more valuable on international flights. They are hard to use on peak travel routes however will work with careful planning and timing.
There is no additional charge at Delta for riding in first or business class. If there are seats in the front you will be boarded there. Domestically on any route with business travel it's unlikely you will be moved up as the FF upgrade list sucks those seats up.