You can email any transaction from the system by clicking on the “Email” button in the right sidebar. A log of all your sent emails will be maintained in Communication.
Comments are a great way to add information about a transaction that is not a part of the transactions. Like some background information etc. Comments can be added in the right sidebar.
Like Assignments and Comments, you can also add your own tags to each type of transactions. These tags can help you search a document and also classify it. ERPNext will also show you all the important tags in in the document list.