I don't see anything in Charles' posts that seem to align with what I thought you asked so I could be off on a tangent.

This is called "single sourcing" and it is possible in Word but it requires some knowledge to set up correctly. There are other software products are more focused at this (AuthorIT or MadCap Flare) but that is possibly more because the user base of technical writers do this as their daily job and understand it better. Word can also do it as long as you understand the principles and can make use of macros and fields or content controls.

"Single sourcing" is a catch all term and can be at enacted various levels. For example:
Language localisation: Same doc but with individual words spelt differently eg Australian reader sees colour, analyse but US readers see color, analyze.
Content: As you have described, document contains lots of modules/chapters and you want to produce variants by showing/hiding modules according to which reader needs it (technical user guide vs quick start vs marketing guide)
Output Format: Mobile vs Web vs online help vs Printed vs PDF. For instance, page numbers in the TOC make no sense in Web or help versions but do in print and PDF output.
Theme: imagine day/night view of google maps - same info but different look for different situation/viewer groups

If I was doing this job and working at the Content level as you asked, I would be putting each module into a Rich Text Content Control and tagging each one and then using a macro to show or hide the ranges according to the user's preference and rebuild the TOC before outputting to PDF.

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Andrew Lockton
Chrysalis Design, Melbourne Australia