There are only a few limitations:

Length:

This is not a true limitation, but there are practical limitations. See this page of the Manual for more information.

Content:

A variable can contain any characters you desire. However, but be away that certain characters, such as quotation marks and apostrophes can be represented in Word in two distinct ways using slightly different characters.

For example, compare 'curly' quotation marks “  ” vs. straight quotation marks "  ".

Likewise, compare curley single quotes: ‘ ’ vs. ' and the curly apostrophe ’ vs. the straight apostrophe ' .

 

When processed within a variable, a curly quote or curly apostrophe is returned as 'different' from the same word with a straight quote. Therefore, [Client’s Name] is not the same as [Client's Name].

Because of this fact, you should refrain from using quotes and apostrophes as part of a variable name. As a practical matter, [Client Name] is just as meaningful to the end user as is [Client's Name] and it avoids the curly vs. straight issue.