Symbols must be composed of letters (from the English alphabet) and/or numbers. They are not case sensitive (i.e. "Bush" and "BUSH" refer to the same claim), although the case that was used to create the claim is saved and is used in many places. The maximum length is 16 characters. Symbols will normally be restricted to about 6 characters unless they are conditional claims (which will be composed of the symbols of the underlying claims, separated by a special character).
Further information about symbols is available here.