Restrictions on name attributes

I know there is a restriction not to use the prefix "Pset_" for user defined property sets. Are there any other restrictions on names of property sets, properties etc?

The standard uses Pascal Case for PropertySets, using only the characters matching the regex `[a-zA-Z_`]. Is this to imply that user defined names MUST or SHOULD follow this style? Or, alternatively, is it permissible to use any technically allowed character sequence, except "Pset_" at the start of a property set name?