Exclusions are the work, materials, costs and responsibilities that a quote states are not included in its price. They sit alongside the scope of works: the scope says what you will do, the exclusions say what a customer might assume you will do but you will not. Good exclusions are specific nouns, not legal fog. "Removal of the existing concrete path" is an exclusion. "Anything not expressly included" is a sentence nobody reads and nobody will hold to anything.
Exclusions are not the same as assumptions. An assumption describes the conditions your price depends on, such as soil you can dig with a machine or a switchboard with room for a new circuit. An exclusion is a whole item outside the price. Both belong on the quote, and both turn into a variation if the customer later wants that item done.
In practice
The best source of exclusions is your own memory of arguments. Every line you have ever had to explain after a job belongs on the list for that kind of job. For most trades it includes some of these: council or strata approvals, making good other trades' work, painting after patching, disposal of the customer's own rubbish, moving furniture, and anything found behind a wall, under a floor or below the ground.
A landscaper quotes $11,400 for a paved courtyard with garden beds. The exclusions read: removal of the existing concrete path; irrigation; plants, which the owner is choosing; and any retaining wall over 600 millimetres, which needs an engineer. Three weeks in, the owner asks why the old path is still there. The answer is already in writing. Removing it is 14 square metres at $45, so $630, and it goes out as a separate quote for the owner to accept or not. The conversation is about a price, not about whether the landscaper is trying it on.
Keep the list short enough to read on a phone, and put it close to the total, where the customer's eyes are when they decide.
In Quote Desk
Exclusions go in the note under the lines, which travels with the quote in the email and sits on the accept page above the buttons. A short list of plain nouns reads well there. The quote builder page shows where the note fits.