What the system is made of and how the parts relate, at a level of detail that survives the next three months.
Where it runs, how a change reaches production, and what to do when that goes wrong at two in the morning.
The reasoning behind the choices, which is the part that is always lost and always needed later.
What a new developer or a new agent needs to know before touching anything.
Pick the level of involvement that fits, describe your product, and we begin there.