Every card and document has a Notes FactBox and a Links FactBox. They are the cheapest way to leave a message for the next person.
On any record, show FactBoxes (Alt+F2) and find Notes: type a note, optionally tick Notify and pick a user, who sees it on their role centre's My Notifications. "Customer called, delivery must be before 10am" on the sales order; "Do not pay until credit note received" on the purchase invoice. Links: paste a URL (the SharePoint folder, the supplier's portal, a Teams chat). Both travel with the record and appear on the posted version where the base application copies them (sales and purchase documents do).
Notes are not a replacement for a field the business reports on; if every order has a note saying the same kind of thing, that is a field. But for the one-off, they beat email.
