The site visit covers head placement, where line sets and condensate drains will run, and where the outdoor unit can sit. In an older house, expect a discussion of plaster walls, balloon framing, and whether line sets go inside closets or along the exterior in covers.
Each outdoor unit needs a dedicated electrical circuit, so most jobs involve an electrical permit through the Building Department, whose Wiring Inspector checks the work. The department offers online permitting, and your installer typically files it.
After the install, expect a walk-through of the remote or app, filter cleaning, and how to set the heads for winter. If you're applying for MGED's rebate, ask for copies of the model information and invoice the Energy New England application needs.