Access within the System
Always ask about inaccessible areas and how they are to be cleaned. The general rule is if there is not sufficient access to inspect the duct then there isn't access to clean it properly. Sending a hose down the duct will not guarantee that it is clean and this technique cannot be checked.
When access panels are required, ensure they are installed using Fire Code approved methods, most importantly, the access panel should be of the same metal thickness as the duct and should not be installed on the bottom of the duct. When access panels are removed, they must be properly sealed back in a place with a fire rated caulking compound, normal silicone is not adequate.