A plan is a sequence worked out with the assistant and kept: what to do, in what order, and why.
Why it is stored rather than left in the chat
A plan agreed in conversation and not written down lasts about as long as the week it was made in.
Keeping it separately means it survives the conversation, can be returned to in a month, and can be checked against what actually happened — which is the only way to find out whether the plan was any good.
What belongs in one
Anything with more than two steps and a reason behind each. Getting receivables under control, preparing for a lender conversation, working through a pricing change across job types.
What does not belong is a single task. A plan with one step is a note, and the overhead of keeping it as a plan is not repaid.
Plans versus the action plan
Two different things with confusingly similar names.
The action plan in Accounting is generated from your data. It tells you what the numbers say needs attention this week, and it changes as they change.
A plan here is something you and the assistant worked out. It is deliberate, it has a rationale, and it does not change unless you change it.
One is a reading of the present. The other is a decision about the future. Both are useful; confusing them is not.
Reviewing them
A plan is worth reopening on a schedule rather than only when something goes wrong. The two questions that make it worthwhile are whether the steps still make sense, and whether the reasoning behind them still holds — the second changes more often than the first and is the one nobody checks.