Most treatment installs change partway through. A pipe run turns out longer than the survey showed, a wall has to be opened, or the installer finds an old fitting that needs replacing. The paperwork for those changes is usually a change order, and it works as a contract of its own. Read it before you sign the original.
What to find in the contract
Look for a clause that says extra work cannot start until you approve it in writing. Ask who on your side can approve it and what form that approval takes: a signed sheet, an emailed quote, or a signed line on the invoice. If a crew lead says they will just take care of it, you have not received a number you can check.
Ask how each added item will be priced. Some providers quote a flat amount per item, others bill by the hour, and some add materials at cost plus a markup. Ask which method applies to this job and confirm it is written into the contract, not only mentioned in conversation.
Check what the change does to the rest of the paperwork
Ask whether added work affects the warranty, the permit scope, or the performance claim. A part the crew swaps in may fall under a different warranty than the main unit, and a change to the plumbing may need its own inspection. Get those answers in writing before you approve the change.
Before you sign
Ask for three things: the approval method for changes, the pricing method for each type of change, and a written statement of what is and is not covered for work added after the original quote. Keep every change order in the same folder as the original contract. If a change comes up on install day, do not sign a blank form. Ask for a copy of what you sign, with the amount and the description filled in.