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 and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW