Re: First time I have heard of .inc files, what are they?

It is difficult to say without actually seeing the printer driver configuration file (could you provide that?), but one possibility is that those are "include" files, which would be customizations specific to your site/project/etc. There is some information in Help > Contents about this:

%include

Using the %include statement in your printer driver lets you include another
file containing printer driver information. Typically, the include file will not
be a complete “.plt” file but instead will contain one or more records that can
be shared by other “parent” files. Multiple %include statements may be used to
included multiple files. Include files, in turn, may include other files.

Syntax: %include “filename”

where filename is the name of the file to be included. Quotation marks
enclosing filename are optional and may be either quotes (`) or quotation marks
(“). The filename may include a path or extension. If the path or extension is
omitted, the missing values are taken from the MS_PLTR configuration variable.
Configuration variables also may be used, in %include statements, to define
where those included files can be found. For example %include
msdir:papersizes.inc In addition you can use %include $(msdir)papersizes.inc,
which is provides flexibility since you can use multiple variables
$(project)$(user)...

Exceptions

A file cannot include itself nor another file more than once. A file cannot
include a file that would cause the inclusion process to loop back on itself.
For example, if File A includes File B and File B attempts to include File A,
then File B´s %include statement is ignored.

Example

Typically, the include file will not be a complete “.plt” file but instead
will contain one or more records that can be shared by other “parent” files.

For example, the style keyword now supports device-independent units.
Therefore, definitions could be moved out of your printer drivers and into a
shared file, which could be included in all your printer drivers. You then can
modify the style definitions for all your printers by editing this one shared
file.

I have just run into a new plotting configuration I have never seen. The company plot driver is calling (for this example) for 4 .inc files

What are they?

Thank you,

JDJATKHA