Clarification On Postal Tax Stamps

I guess I need some clarification on postal tax stamps. Above is the definition from Wikipedia, but I'm still a bit unclear on how they work. When you purchase a postal tax stamp, are you only paying for the 'tax' and still need to purchase an additional postage stamp to cover the cost of the actual postage? Or, similar to a semipostal stamp, is the additional tax cost added onto the cost of the stamp (in addition to the cost of the postage)? If it is the former, then they are really revenue stamps and can't be used as postage by themselves.

If, on the other hand, they are "similar to semipostals, except their use is mandatory instead of voluntary," they could be used to pay for postage. If that's the case, were they sold at a higher price than the actual cost of the required postage (like a semipostal?).

Thanks for any help.

An example in case are the German "Notopfer" stamps from the last century. They were basically postal tax stamps because they were required on letters ( mandatory ) but did not pay for postage. You had to pay a certain amount or the stamp in addition to the regular postage.

Peter

yes, the postage is separate.

Serbia issues 4-6 postal tax issues each year. They also take it to the extreme for the Children's Week. Beside the normal postal tax stamp, they issue 1-3 self-adhesive labels (same/similar design) which is used as a tax on entertainment events (movies, concerts, athletic events)

AskPhil :

Postal tax: stamps used on mail, for a specific period of time, in addition to regular stamps.
Postal tax stamp: a revenue stamp required during certain calendar periods with proceeds going for a specific purpose or
charity.

Thanks, that helps a lot. So, they are actually more like revenue stamps than semipostal (charity) stamps.