it’s the contract !!!
and from the simplest to the more complex projects, your client can find pieces of work that for your perception should be “out of scope” but from their perception is “in scope”.
For sure, perceptions are not contracts, but they can turn in unnecessary discussions that could disturb your client confidence. Apart of the extra-efforts you will have to do to explain that “this” is out of scope.
I have seen this week how a project manager was giving explanations that could be solved with a better “out of scope” definition.
In this case there has not been major problems, but in other cases you can loose a big piece of credibility, so we have to take care.