A placement plan is the document that decides whether a campaign is measurable or a guess. It is short, and reading it properly takes ten minutes. Four columns carry almost all of the risk.
The site and the slot, not the category
A plan that says 'business and finance sites' cannot be checked afterwards, because nobody can say whether it was followed. A plan that says a named site, a named position and the size of the unit can be checked in seconds. Insist on names, and ask which slots were considered and rejected.
The sizes you have to supply
Every placement needs creative in specific dimensions, and a campaign cannot start until they exist. The plan should list them, so the work of adapting artwork is visible before the start date rather than after it. If a size appears that you cannot supply and cannot adapt, it should come out of the plan rather than delay the launch.
The delivery the budget should buy
This is a range, not a promise, and it should be stated as one: so many thousand impressions, or so many views for video, or so many sends for a newsletter. It is the number the campaign is judged against when it reports back. A plan with no stated delivery gives you nothing to check.
The dates you will hear from us
The reporting schedule belongs in the plan, not in a conversation. Weekly and monthly reports are different products and should be priced differently. Once the dates are written down, a late report becomes an obvious fact rather than a matter of memory.
What should be missing
Look for the placements the plan refuses to buy, and the reason. A plan where every placement is recommended reads like a sales list. A plan that says a cheap slot was rejected because it sits below the fold, or that a category was skipped because the audience does not index for it, is a plan written by someone who looked at your situation.