The abdomen is a cat's most vulnerable zone, so exposing it signals relaxation and trust, but it is a display rather than an invitation to touch.
For most cats, belly rubs trigger a defensive reflex where all four limbs grab and kick the hand, instinct rather than betrayal; exceptions exist but should not be assumed.
Respond with praise or chin and cheek scratches instead, treating belly time as a reward to respect, which makes cats roll out for you more often.
Belly shows also occur in play excitement and heat, so read the context; sudden frequent rolling with itchy skin should be checked dermatologically.
