@pemerton responded with "Cunning Expert d8" but that doesn't give the whole of the mechanic. Rather that's shows some relevant parameters of the mechanic. I don't know MHRP but I do know the Cortex System that it is based on.
Stripped down
Everyone writes down their character’s name and some things they’re good at doing, they may also have a metacurrency
Generic things a character are good at doing are represented by traits - descriptive labels with dice ratings attached to them (such as “Cunning Expert d8”)
Games like MHRP will structure traits into sets, which could include attributes, affiliations, distinctions, powers etc. Sets of traits have defining qualities that condition where and how they apply.
Traits are diegetic: if my character is a cunning expert, that's something they and others in the world can know about. I can include the die in my roll whenever it makes sense that it applies. I can't include the die in my roll where it doesn't make sense (and sets generally make that even clearer.)
A metacurrency -- in it's generic form, "plot points" -- is earned by players when they roll 1 on a die, and can be spent by them to add dice to future rolls. GM gets plot points too, but the generic way GM uses them is to introduce NPCs.
Games like MHRP can alter the way plot points work in the game using mods. A doom pool is a mod that swaps GM plot points for a dice pool.
And finally, when you want to do something and there’s something that might get in your way (such as the environment, another character, or time), you make a test.
Someone else picks up dice to establish the difficulty number you must beat to succeed.
You assemble your dice pool, roll it and keep two dice to compare with the difficulty number.
You may also nominate one die you didn't keep to be your effect level (the more sides the better, e.g. d12 is a stronger effect than d6 regardless of what was rolled on it.)
Any 1s, whether you keep them or not, create complications (and earn you plot points.)
In specific games, there can be numerous concrete details that make all this specific to some imagined world rather than generic. For example, a distinction "Your Life Before" may let you add a die to a crisis pool to double your attribute die when you connect to your old life.
This general apparatus is tremendously versatile, and at the same time strongly
diegetical. I say it is diegetical because every element of the game mechanic associates with something diegetic in play. If Cunning Expert doesn't matter in this situation, I can't include that d8 in my roll. If it matters, I can include it. If I include that d8 in my total and beat a test I can narrate that it made a difference. There's more to it of course, including an absence of assumptions some might port into it with their set of unwritten rules.
I think one could complain that it is not
para-diegetic, meaning that the process itself is not set up to unfold in a sequence that feels like some imagined causal chain. That makes sense, because rather than a collection of sub-systems each bespoke to some phenomena significant to play (potentially and usually hung off a backbone system), the core apparatus is applicable to any phenomena that becomes significant to play.
I've observed that the quality of being diegetical matters to most who favour process-simulation, while the quality of being para-diegetic matters to some more than others (and doesn't in my opinion turn out to be all that robust... it typically relies on glossing over deviations.) Folk comfortable with traditional game system structures might count Cortex out from being proceess-sim just because it doesn't feel like those traditional structures.
