X match dating
These terms are used to describe how your X chromosome SNP markers match another's.
The matching tools (such as GEDmatch) will compare the single X chromosome of a man to both X chromosomes of a women.
This can result in an entire family line where you will not have any X-DNA matches. Some studies have suggested only 3% of mother-child events don't have X chromosome recombination, though analysis of DNA test results suggests a slightly higher rate.
The son would be a 100% (196c M) half match with his mother, but a 100% (196c M) full match with his mother's father.
It is possible (though quite rare) that you could be a 100% match to someone many generations back - and as such, you can share notable X-DNA with a very distant cousin.
As you can see, if you're unable to test grandparents (especially maternal grandfathers or paternal grandmothers), X-DNA data will be less useful to you.
You can compare X-DNA data for siblings and/or cousins, but piecing together the puzzle is difficult due to the variability of the X chromosome.