Ars Magica spell durations such as ‘sun’ (spell expires at sunrise or sunset) would be a simulationist mechanism from my perspective as it corresponds to a real thing in the game world. A spell duration based on time (even if that time is expressed in rounds which are in turn expressed in seconds etc) is also simulationist since it is based on real passage of time. But a mechanism where an effect is based on an ‘encounter’ doesn’t map to a real-world construct, or even an in-game-world conept that I can see. That would be a gamist mechanism, or maybe a narrativist one.
These things aren’t ‘bad’, often people prefer them since whether a spell lasts 4 minutes or 5 is arguably less important than whether it lasts to the end of the encounter. But in a strongly simulationist system it would be a firm definition. And the benefit of this, to people who appreciate it, is that you can then make plans based on those rules and those plans have meaning. It’s an example of
Sanderson’s First Law of magic, but it can apply to almost any endeavour in a simulationist game system. The players can make plans based on in-game-world-logical inferences.