If a signature line has an X and then "his mark" or "her mark" then that means that the clerk wrote out the name on that line and the individual made the X.
"If a signatory was not literate, they would often mark an 'X' instead of a signature, which is referred to as a mark. Similarly, you could sign whatever you like (Jane Doe could sign 'Jane', 'J. Doe', 'Jinny', or 'Bob') and it is still considered your 'mark' if that is the way you always sign it. There are still very many illiterate people now, but most have learned to approximate their own name at the very least. The job of the illiterate in this modern world is to hide their illiteracy at all costs! – Loquacity May 7 '11 at 23:56" Taken from http://english.stackexchange.com/questions/24283/your-signature-vs-your-mark
This is why we should always index all name variations with “or” and do not always just index from the signature line. FamilySearch Indexing