This is a "beta" version of the game. You need to explicitly set your profile in Steam to play on it. This branch of the game is updated frequently and it gets all the new features many weeks or even month before the mail (stable) branch. But how the name implies, it's for experimenting. And the mods get broken pretty much often due to frequent updates to the game codebase.

This mod always follows the experimental branch. While the same version of the mod can be used in both branches, you can use it either way. When incompatibility is introduced, the new version of the mod will only be published for the experimental branch until the branches come in sync again (it depends on the game devs).