Oval carbon saddle rails compatibility
7x9 is the size that tends to be used when they're trying to make it compatible with "any" seatpost. Normal metal rails are 7mm, so making it a 9mm oval/rounded rectangle fits in in most cases. It's kind of a hack solution that's become normalized. If you look you could find seatposts where a 7x9 will give either iffy thread engagement or interference/awkwardness with the clamp, although neither are common issues by any means. And of course posts exist that clamp the sides of the rails, and you need to avoid those. What you do want to look for is a post that's got nice abundant contact area with the rails and isn't going to press on them with any sharp edges. You can find posts that are counterexamples of this, mostly older basic ones, and thats what you want to avoid.