When a person is tested for X& Y variations, many cells are examined. When this test shows that a person has some cells with one number of chromosomes, and other cells having more or fewer chromosomes we call this being mosaic.
A person can be mosaic with any X or Y variation.
Some people with Klinefelter syndrome have 47,XXY/46,XY mosaicism. While some small studies found those with mosaic XXY have less severe symptoms that those who have XXY in 100% of their cells, we really do not know for sure. Note most people with XXY are not diagnosed during their lives, and XXY is a spectrum condition.
To understand how being mosaic for any X or Y variation may affect you or your loved one we strongly suggesting seeing a genetic counselor or a genetic medicine specialist.