For OHD? A paid community suppor version that the gives you access to occasional playtests and lets you try the indevelopment version. There is also some singleplayer DLC I haven’t tried.

99% of the time I play the free version, there are several modded servers that have consistent player numbers and the free version is obviously where the playerbase is so you really don’t have to spend money on OHD at all, the monetized parts are more presented as ways to support the developers and community if you love the game.

Idk, the developer wanted to make Project Reality but in a modern game engine and yeah… that is what it basically is and personally I love it.