<...>
Feature restrictions do vary according to rank and/or activity as you say. @John (John K.) has a post where these restrictions are described. The post was created in 2013, but has been kept up-to-date through 2018 (see
Forum ranks/positions/badges (What do those shiny coins under my name mean?)).
Formal restrictions per rank vary in terms of signature (size, fonts, etc.), posting (cool down time between posts, PM limits, etc.) and avatars (need to be at least a Full Member). Aside from that, Campaign Managers may set restrictions based on rank, earned merits and posting quality (the latter unfortunately is rather an exceptional thing though).