assuming you have T0 and T1 many of them the T0 gateways will be like you ISP CPE and the T1 like the tenant router, T1 is only for having the ability to provide services and tenancy between many networks connected, another reason is that if. you lost all the T0 the networks attached to T1 won't be affected at all to communicated between them.
So T1 "can't " be directly connected to physical networks unless you have the NLB T1 stand-alone case.
I hope this helps.