What does mandatory beta mean?

[Uthgaard]

Anything and everything. Just play it. With a lot of people playing regularly at all levels, we're more likely to discover random bugs. If you desire direction and want to test something specifically, read the announcement and test the most recent changes. Post any issues to the forums. If you're not sure if something is a bug, post it anyway. It will get sorted out by one of us or your peers.