[EvanAnderson]

I went down this rabbit hole this morning myself. There's a nice document at Bitsavers (http://www.bitsavers.org/pdf/bbn/imp/BBN1822_Jan1976.pdf) that has details about the two different IMP models, and the TIP and Pluribus IMP (an interesting multiprocessor IMP that I'd never heard about prior to today).

The TIP was the "Terminal IMP" (per the linked document) that provided connections to 64 terminals directly to the network through the IMP (with no intervening host).