What is Parent unit?

A parent unit is the part of a monitor you keep with you, holding the screen you watch and the speaker you listen through. The camera or sensor in the nursery is a separate piece, so a monitor sold as a parent unit still needs its camera side to be set up and placed.

Here the listed brands are the same on this point. ADC Solutions Auto, Amazon, Arenti, babelio and Baby Monitor Direct each state that a parent unit is included. So the screen you hold is not what separates them, and it is worth looking at screen size, night vision and range instead.