Crit or Haste Cap.
There isn't a true haste "cap" for disc either. There are haste breakpoints for disc, but they're quite minor, even more unimportant than the ones for holy and shadow. The 12.5% (raid buffed) one gives a 5th tick to renew, divine hymn, and hymn of hope for the same mana cost. Renew is awful for raiding disc priests, but divine hymn will absolutely be used next patch where its buffed by 66%. You'll likely be over that passively, and note you can bubble before divine hymn/hymn of hope to increase your haste for that spell.
The meaningful haste "soft cap" is when your 1.5 second cast time/instant spells hit the 1 second gcd minimum. At that point haste devalues quite a lot as a throughput stat, but is still quite good for PoH spamming. You may be over this with heroism/bloodlust, but your throughput should be fine then so this is ignored. I guess you could stack enough haste in 4.3 with 3% from talents to hit it under borrowed time, but that would take a lot of work.
TL;DR: For a new-ish disc priest, pretend haste/crit caps don't exist. They aren't reachable in normal play, and the breakpoints are minor enough you won't notice them unless you hunt for them.
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