This doesn't work everywhere on all collections. For example, if I am looking for the software version and hotfix currently running, the select has no effect here:
curl -k -u admin:myPassword -H "Content-Type: application/json" -X GET https://10.19.1.10/mgmt/tm/sys/software/status?$select=active,version,build -d "" | python -m json.tool. Nothing gets filtered. I get everything