Prospective tenant "has Tax ID but no SSN". What does it mean?

[Carlin A Guarneri]

Hello, 

Prospective tenant informed me her mother who will be living with her (and submitting an app as well) "doesn’t have a social security actually it’s just a tax ID." I've not heard of this before and was hoping to group might be able to shed some light on what this might mean so I can be informed while I wait for their apps to come through tomorrow. 

My understanding was Tax ID's were allocated for businesses, not individuals? Hoping for your insight.

Thank you in advance,
Carlin