What is the name of Network Setup Assistant's window? This is a somewhat common interface, and is generally used to move a user through a series of sequential operations. It can be found in Apple's Network Setup Assistant (System Preferences > Network > Assist Me > Assistance), in many third party .pkg installers, and in applications like VirtualBox. 

It has a Low Opacity (light) image as a background, with a translucent Text Field on it, with a visually distinct bottom row with the next and previous buttons.

Here is an example:
These kind of windows are commonly called wizards, or specifically in the context of macOS, a term you already mentioned: setup assistants. That term actually describes the entire sequence, so you might want to call a single screen a wizard screen, wizard step or wizard dialog instead.

The transparent background image is something specific to macOS; most Windows installer screens show images as well but there, they're confined to either a column on the left or a row on top of the screen.