It's about filtering posts submitted by users on Tagmask. Tagmask is a community site for programmers. So, content should be created by users. It's a problem of chicken and egg.

> How are tags assigned/chosen?

If you submit a post you should give it correct tags. For example, if you submit a post about some algorithm on PHP you should attach to it tags "php" and "algorithm". And in this way any user who subscribed to tag "php" or "algorithm" will see it in his feed.