The SITE virtual field allows you to limit the search results of a given site. This example describes how to narrow down your search results to a single site called mysite:
test AND SITE:mysite
You can limit the results to any site. To do this you need to use a special site value _ALL_SITES_, for example:
test AND SITE:_ALL_SITES_
You can use the _EVERYTHING_ special value when the SITE condition should be ignored, for example: test AND SITE:_EVERYTHING_