She's long in the rib cage - so her span of back is getting good support. The withers reach well back as well, which offers good support at the front end and her LS joint is well placed - this is why I think her back is long, but not in the loin. Mares tend to have a little bit of a longer back, nothing wrong with that so long as it is well supported and the loin is strong and the horse isn't going to be really big.

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For those of you who have very little information,
but pride yourself on drawing really big conclusions.