The note attribute simply allows you to put in a simple comment to help organise and idnetify your code. The note could, for example, give a brief inidcation as to what the flow-function does.
<instruction xmlns="http://www.obyx.org" note="Display Hello World" > <input value="Hello world" /> </instruction>
Note attributes are included in xpath traces - so error reports will show notes as a part of the xpath. This is redundant, but useful for manually finding the breakpoint in the source code.