As I understand, the 'liner' is the internal guts of the ventilator, basically everything but the outer shell. The outer shell in this case is your wood hood.

Baffles are considered better than mesh. Free-flowing.

As to 42 vs 48, it depends on many things. What size is your cook top? If you go with smaller and your wood is 48, does Z provide something to fill in the extra space (like a sheet of stainless)? and do you need a wood piece cut to attach it?

A liner or insert may be thought of as a hood built without concern for its external appearance, as its external surface is hidden by cabinetry. Either the insert has to be designed for an existing cabinet, or the cabinet has to be designed for an existing insert. The insert has to protect the wood it is within. It is possible that a mismatched cabinet and insert can be adapted to work together; [the devil is in the details, but so is salvation. -- H. Rickover].

And insert can be relatively passive, comprising just baffles and housing, or active and include a blower. Without a blower it is ducted to an in-line or external blower.

kas