How can a master document pass information to linked documents?

I want to have a set of chapter files using the same template, that include at various places (especially heading text) a field of text set in the master document. The master document has to be able to redefine this text several times so groups of chapters get one phrase while others get different text. I imagined I could define a field in the master document whose value would be seen in the linked documents that come after it, until (somehow?) it is changed further down the master document to define different text for the next few linked files.

If I cannot do that, and need to have different templates, with different text at those spots, is there a quick way to change the text in all the chapter files linked in a particular part of the master document?

Background: I want to have a book with three parts (and then appendices) with all chapters that are in a given part having some differences: firstly stylistic changes e.g. red, green or blue for heading backgrounds - which I think I’ve worked out now, but also: some places in the text that differ in a way that should be up to the master document to define and have that choice ripple through to everywhere it is needed. To add to the complication, the chapter “numbers” I want are letters, in order but with gaps (e.g. I don’t have a Chapter “O”, it skips from N to P) in case that affects any solution based on a database where the chapter number is a key to the data I want.

Information passed from master to sub-doc: this works but is limited in functionality. Please edit your question to describe precisely the kind of information you want to pass (numeric, text, heading, cross-ref, …?)

Numbering gap: this is not possible with the auto-numbered sequences (chapter numbering or list numbering) because you have no action on the counter (it is a monotonous integer sequence which is then converted to an external representation, i.e. numbers, Roman numeral, alphabetic, …).