Imagine the request:

• Visitors where Custom variable 1 IS NOT "test".

I think this segmentation request should match and include in the dataset, the visitors that had a Custom var1 "hello" or "type" but also those that did NOT have a custom variable 1 set.

This ticket is about changing the API so that when EXCLUDING visitors, we dont exclude those for which the value was not set.

If you wish to also exclude the visitors for which the value was NOT set, then you can use the IS NULL match eg. referrerKeyword==;browserCode==ff will select all visitors that use Firefox and did NOT Have a keyword set. see #3932