Properties
Each property is its own record (settlement date, ownership, loan, and everything you've imported against it), and your portfolio view is the sum of all of them.
Every property you add becomes its own record: address, settlement date, ownership structure, loan details, and every transaction and document you attach to it from there. Your portfolio view is a roll-up of every property you've added, so the accuracy of the portfolio-level figures depends on each property being set up correctly to begin with.
Editing a property
Purchase details, loan terms and ownership can all be updated later from the property's own page, which is useful when you refinance, or when a fixed-rate period ends and the rate changes.
Selling a property
Recording a sale keeps the property's full history in your reports rather than deleting it, which matters for CGT: you want the record of what you paid, held and claimed, not just a clean slate afterwards.
See adding your first property for what each setup field feeds into.