"Pro" is the tier of how much you spend each month to access tokens, while "xhigh" is the setting for how many tokens are spent on any given answer.
(Unless I've missed OpenAI having another ill-advised naming decision to confuse things again, like they did with 4o and o4 models existing at the same time).
Still, you insisting did at least let me find it: https://developers.openai.com/api/docs/models/gpt-5.5-pro
As opposed to https://developers.openai.com/api/docs/guides/reasoning#reas...