There are no IRC requirements for spacing wood sheathing that I am aware of. However, it is considered good practice to leave a 1/8" gap between sheets, and that is also “recommended” by APA … but it is not required.
Actually panels marked “Sized for Spacing” simply mean that they are slightly smaller to allow for the recommended 1/8" gap (see attached APA TT #055). It doesn’t mean that panel edge clips (“H-Clips”) are required … although there are types of clips that will help with properly spacing the panels.
Panel edge clips (or some other type of edge suppoprt like blocking) are only required for structural ratings with thinner panels and larger support spacing … like 3/8" sheathing on roof trusses 24" o.c. would require panel edge clips for support (See Item #7 under “Fastening Hints” on Pg-2 of the attached APA Guide #N335, and Footnote-d of IRC-Table R503.2.1.1-1 on allowable panel spans).
Hope that helps …