A simulationist design/writing is an approach where the designers/writers make rules and design content as if they were diegetic in the world. Not necessarily one-to-one all the time but works as if 'it exists' in the world
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A simulationist playstyle is one where the GM and group focuses on the interaction and exploration of the 'game world' itself. The players at the table want to act as if their characters actually do exists in an imaginary space-time location
Most people can probably agree that RQ, and RM, are designed to support a simulationist approach to RPGing. But they contain plenty of rules that are not "diegetic" - rules like *each player, to prepare for their participation in the game, must create (or otherwise obtain) a player character"; and rules about how the GM is in charge of framing scenes and establishing certain consequences of declared actions; etc.
If we focus just on action resolution rules, then we do find many rules that are intended to represent processes or events that occur in the fiction. But not all of them: for instance, Rolemaster has rules about open-ended rolls. These rules, which are rules for generating random numbers with no upper bound although an increasingly low probability, don't represent anything in the fiction.
If we focus on PC build rules, we can see a difference between RM and RQ that is quite interesting. The principle mode of PC progression in RQ is via the method of
use a skill, roll over the skill rank on % dice at the end of the session, increase the skill bonus by a modest amount. This represents an in-fiction process, of getting better by doing.
In Rolemaster, PC progression is via (i) allocating a level's worth of development points across various skills, with costs determined by character class (ie the character's entrenched disposition and capability, based on aptitude and training), and (ii) actually earning a level's worth of XP, so that the planned improvements actually crystalise. There are various XP rules to be found, but the core ones are what I call
learning based on hard field training - ie you earn XP by putting your skills to the test in real challenges (and mechanically, this is expressed as gaining XP by giving and receiving hits in combat, using your skills to overcome challenges, and even dying and returning to life).
The representational character of the RM PC progression rules is clearly weaker than in RQ. For instance, no
more effort or success in fighting is required for a player to advance their PC's weapon skill by two ranks rather than by one rank.
When it comes to the notions of
interacting with the game world, and/or
players acting as if their character do exist in an imaginary place, for me that is different again. For me that is just
roleplaying in a serious way. I've done RPGing which isn't serious in this sense - mostly pretty light-hearted AD&D play - but most of my RPGing fits under this description. For instance, when playing AD&D dungeon crawling in a less light-hearted way, the players and GM are pretty closely focused on the "game world" (that is, the fiction of the dungeon); though the
characters are generally just pawns/ciphers. When I play Burning Wheel, I am very much acting as if my PC is an (imagined) person in an (imagined) time and place.
If this is sufficient for play to count as "simulationist", then nearly all my RPGing is simulationist. My group's play of 4e D&D, for instance, would count as simulationist by this particular measure.