no region limit's processing outside by design. process used when you need to submit not the subtree but subset of inputs. Sometimes you can't limit just subtree because some components should be excluded from processing in that subtree. Real case: when two inputs generates the suggestion for the third which has validation plugged.
submitting subtree - will cause input 3 to be validated also.
So you could use at input1 ajaxSingle="true" and process="input2" and only two fields will be processed and the validation at third will not be fired..
Ok, that makes sense. Thanks!