The "field" is filled by default based on your posts. The more posts the higher the default "fill" (Senior member, etc).
You can visit your UserCP menu on the green menu bar and change it.
Rules do apply to what is "OK speech" to put there. Basically, if it is OK to put in a post it is OK to use under your avatar (photo).
You can't mislead (by putting "Moderator" or "Admin" there for example, unless you are one.)
Lou, Laura, & Freya the wonder dog
2008 GMC Sierra 3000HD Crewcab SB Allison Duramax
2019 Flagstaff 8529FLS - Pullrite 3300
HAM CALLSIGN - KC3FFW