That's the analog input block.
It has several dedicated inputs.
I1 = wideband input
I2 = map sensor input
I3 = egt input
I4 & I5 are for other analog inputs for example. Some inputs from Translator Pro, or inputs from pressure transducers, etc.
There's also an input labeled scn. for the scanmaster hookup, so you can monitor such things as boost and wideband on your scanmaster display.
As for mounting it. Most find a place behind the glove box. The options for mounting it are endless, and up to you.