The private brain
Notes is where everything you know that is not public goes. What was said, what you agreed, the document someone sent you, the thing you noticed and want to remember.
You can upload documents, mark what matters, add tags, and say who a note relates to. After a conference you will often have dozens, each attached to the person it belongs to.
Notes that do something
A note can be improved by O in one step. It links the note to the people in it and proposes what follows, such as a reminder to follow up next week. Accept it and the reminder exists. The note stops being a record and becomes the next action.
How it compounds
This is where the memory actually grows. Every conversation, every note, every document lands in one place and stays there, and O builds a picture across them of the concepts and people that keep coming up.
The more you use it the better it gets, which is the opposite of how a contact database behaves. A database decays from the day you stop maintaining it. This accumulates.