All of the theme park restaurants are very casual - theme park attire is fine. Shorts, t-shirts, etc. is what most people wear.
Victoria and Albert's is dressy - men must wear a jacket.
Several of the signature restaurants - Artist Point, Cali Grill, Jiko, Narcooses, Citricos, Yachtsman Steakhouse are the ones that come to mind - require "resort casual" clothes. This is generally taken to mean nice shorts, shirts with collars, etc. They did just redo the policy to allow t-shirts, so probably anything other than tank tops, caps, swim suit cover ups, etc. is fine. When you make an ADR for one of these places, they'll remind you. Maybe someone else knows the exact guidelines...