I've heard many words used interchangeably with "NIU". And like Ed's beef with the RJ-45 bastardization, I understand what people are talking about (whether they correctly use/misuse or slur the term "NIU" to mean):
NID
Smartjack
TNI
network or loop termination point
loopback box
Demarc
customer interface
card nest
mount
ONT
beige box with lights and Adtran/PairGain thingies in them
The truth is that an NIU terminates digital circuits (T1,PRI or ADN) only, though I would also deem the 2/4 wire converters or HDSL/SDSL term points as nearly the same. And while an NIU is a demarc, the term "demarc" is rather general and can refer to all kinds of terminations.
An NIU can be placed into loopback via remote commands, but it is not the same thing as a repeater (and don't get me started on telco techs who accidentally loop-up a repeater while thinking they're on the Z end).
"Customer interface" is OK I suppose, because it IS just that. It just strikes me as a description so generic that it always begs the question as to what service and/or facility was ordered. "Customer interface" covers everything from an old wire-wrap block to a piece of RG-6U w/an F-connector.
