It is possible to add notes to any pipe/duct section in a Netsys project.
This can be achieved by selecting Notes>Pipe sizes>Add some, or Notes>Pipe properties>Add some, and then clicking on the desired location.
The above methods will only work if the correct pipe height has been set.
For example, using Pipes>Pipe info shows that the below pipe is at a height of 0.35m:
Then, this same height needs to be set in the Pipes>Defaults menu:
The properties will then be displayed after clicking on the Notes>Pipe properties>Add some option, and then selecting the pipe: