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The Law of the Fiction
It's important to remember that all the mechanics in these core rules are subordinate to the fiction. When we write the words "the fiction," it's an obnoxious shorthand for the imaginary world that you and the GM create together during the conversation.
Here's the law of the fiction:
What is the Fiction?
Each fictional thing in the imaginary world bears a relationship to all other fictional things, and these relationships have to logically cohere in order for the scene to make sense for everyone involved. This is like the laws of physics for imaginary worlds: without them, we can't all participate in the same theater of mind because we can't all agree on what's happening. Put another way, a mechanic can't resolve an action if the position of things in the fiction do not make it possible for the action to happen in the first place.
What is Implied?
Here's an example: If you want to attack your enemy with an ax, you have to be capable of holding an ax, and your enemy has to be within reach of you.
These things might seem obvious on the face of it, but nowhere in the rules of OSR+ is it spelled out that you have to have hands to hold an ax, or exactly how far away you need to be in order to strike someone with an ax. These things are implied by our imagining the relationships between axes, hands, and opponents. And so in order for a mechanic to resolve uncertainties in the game, it must first be possible (according to these fictional laws) for the action to be resolved: i.e., if I don't have hands, I can't hold an ax with which to attack. We rely on the "position" of things in the fiction to suss these things out, and then the GM applies relevant mechanics to simulate the outcomes.
What is Assumed?
A better example: consider the rules for attacks of opportunity. The rules described are entirely situational. You aren't entitled to an attack of opportunity unless you position yourself in the fiction such that an attack of opportunity makes sense for the GM to grant.
Another example: the burning status. The burning status doesn't explicitly forbid you from moving or acting on a turn if you're on fire. All the rules do is simulate the reality that an unattended fire tends to build up cumulatively the longer it's left to consume flammable things (hence the cumulative damage it inflicts). But the fact that you're on fire positions you to put yourself out. If you decide to ignore that fact, this raises questions as to how and why, and so the GM is warranted to break the rules in order to intercede on behalf of the fiction.
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