In a SoapUI NG response viewer, XML format, what does Alt-V Validate?

This "Validate" feature ensures that the request and/or response (whichever pane you have open) is legitimate and correct when compared against the appropriate WSDL/Schema/etc. 

So if you had a request that had an element that had to be an integer value, and put in 32, and validated, it should validate against the schema.

If you instead put in 32A, since that is not an integer, the validate should return a schema validity error.