What
Effort is how hard Claude reasons before answering. The description of the xhigh level now reads "Deeper reasoning than high, just below maximum (on supported models)". Before, the text in brackets was filled in with a specific value. The check that decides whether xhigh is supported was also reworked.
Why
The visible change is cosmetic, in the list where you pick an effort level.
Something disagreesSomething we can check disagrees with this entry, or the writer said they could not settle it.
The writer flagged doubt
Whether the reworked check changes which models allow xhigh is not known.