To use programmer speech for a moment, it seems to me you're measuring rigidity and fragility of game rule systems. To wit, "rigid" refers to tightly coupled rules where change in one impacts the other. "Fragile" refers to complexly coupled rules where change in one impacts another seemingly unrelated part.

For any given system, you may be able to sort out which parts are rigid and which are not, or if you're making a new system, you can decide ahead of time to make it modular so it won't be very rigid or fragile. But on a general level, you're still talking a distinction between system and parts of the system. "Content" hence just becomes a name for the parts of the system you can swap out.

You can compare it to older distinction between "crunch" versus "fluff". Practically, that distinction means one of two things: either it's "mathematically expressed rules versus rules expressed in a natural language" or it's "rules I care to enforce versus rules I don't". People often conflate the two because they find it easier to invent new natural language expressions than mathematical expressions, so it feels like it's easier to swap out natural language elements, and consequently, those elements feel less important, less set-in-stone. Actual system level analysis may prove such distinctions entirely arbitrary.