It’s common for someone to make “their mark” on a document when they are not able to sign it. It usually means the signer is illiterate, but they also could simply be unable to physically sign their name.
This document, from the probate file of Benjamin Butler in Vernon County, Missouri, contains a request from the widow to have another person appointed administrator of her husband’s estate. Fortunately this document clearly indicates Nancy Jane is the widow of Benjamin Butler.
What’s a little unusual is that “her mark” is referred to as “her cross.” Personally I don’t think there is any real significance to the use of the phrase and it probably was simply the personal preference of the writer of the document.
But it’s always interesting to note when a word or a phrase is not the one a person would expect.