Your question is so broad as to be unanswerable. What you might consider an 'extra special common' function may be something I think of as 'expected normal function'. You might as well ask the same question about Excel or Word or Ubuntu...too undefined.
Bottom line, Notes is a development platform and, as such, you can make it do whatever you want using one of several languages and sub-development environments available to you.
If you have specific features you're interested in, please ask.
With regards to watermarking documents, you can do this by adding an image to a form design. The image can be there all the time or computed based on conditions you set. You have an option to force use of a specific form in views or via a 'print' button you create and add to the design. Depending on your intent for the watermark, this may or may not meet your needs.
Hope this helps a bit.
Doug