XY repeatability works like this: I start at some point in XY space, say 100, 100.  And then execute a series of random moves of the print head all around the build envelope.  When I’m done, I tell it to return to 100, 100 and measure how close to the original point it is.  The closer we are, the better XY repeatability we have, and the better our parts will look. Poor XY repeatability can make the layers look slightly misaligned from one another.