XCJ10 is simply the jumping clear rate from your horse's last 10 cross country rounds.
How to calculate it:
A clear round is any round you finished with 0 jumping penalties. Do not count time penalties.
If you started
the XC phase but didn't finish the course, that counts as an unclear round.
10 previous runs is best, but you can use as few as 3.
Check out this article to see how to calculate all your horse's simple metrics.