The size of a business buying group is quoted often enough that it has stopped being surprising. Gartner describes groups running from five to sixteen people, drawn from as many as four functions.
The number matters less than the distribution. You will meet one or two of those people. The rest will form their view from material that reaches them second hand, usually forwarded by the person you did meet, usually without you in the room to explain it.
What survives a forward
Anything that depends on delivery does not survive. A demonstration, a story told well, a reassuring manner and a persuasive answer to an objection are all lost at the point the file is attached to an email.
What survives is what was written down: the scope, the price, the limits, the method, the definitions, the exit. Those are read by people who have no relationship with you and no reason to extend the benefit of the doubt.
The finance lead reading your proposal cold is not evaluating your firm. They are evaluating a document, and a document that is vague about what is excluded reads to them exactly as it would read to you in their position.
A test worth running
Take your standard proposal and give it to somebody who was not in any of the meetings. Ask them what is being bought, what it costs, what it does not include, and what happens if it goes badly. If they cannot answer all four from the document, the members of the buying group you never meet cannot either.
Most proposals fail on the third and fourth questions, because those are the ones a document under deadline leaves out, and they are the two that the people outside the room care most about.
What we take from it
Writing for absent readers is a discipline rather than a style. It means naming the limitation before it is discovered, defining the term rather than assuming it, and putting the commercial facts where someone skimming will find them.
It has a second effect inside your own firm. A document that has to survive a hostile cold read cannot rest on things you have not decided, so producing one forces the decisions that vague proposals exist to avoid.