...Mark important objectives, NPCs, items, traps, etc. that are vital for understanding the player experience. But don't clutter the drawing with too much...

...stand inside a shop, almost get killed by assassin NPCs
Follow: secretly follow assassin NPCs through the city, back to their employer
Mansion: sneak...

...Sound design, HUD / UI, and NPC AI as speed metrics
The brain processes all senses together. Vision isn't just a matter of seeing...

...control A
Fight
PvE territory
For more on designing with NPCs and AI, see Encounter design.
For single player / PvE game modes, territory is...

...is the monster "unable to navigate water due to broken pathfinding AI", or is the monster "afraid of water"?
Minimal worldbuilding
For most level...

...For example, if a designer didn't like a certain texture, sound, or AI behavior, then they could simply omit it from their levels...