A unit is the rentable space inside a property
"Main Floor" in your duplex. "Unit 2" in your triplex. "Entire House" if you rent the whole thing to one household. A bedroom, if you rent by the room. The test is simple: if someone can sign a lease on it independently, it is a unit.
A single-family rental still has a unit. It just has exactly one, usually called "Entire House". That feels like paperwork for its own sake until you buy a second property and want one consistent way to ask what is rented and what is not.
Why the unit layer exists
Because a property cannot answer the questions you actually ask. Your duplex has two tenants, two leases, two rents, and two very different maintenance histories. "The duplex" is not a useful answer to "who is behind on rent".
Units are what make occupancy real. Vacancy is a unit-level fact, not a property-level one. So is rent, turnover, and the timeline of who lived where. A rent roll is that view written down: one row per unit, occupied or vacant, with the rent beside it.
What a unit is not
It is not a lease, and it is not a tenant. The unit outlives both. Tenants move in and out, leases start and end, and the unit stays put with its history accumulating underneath it.
That distinction is what lets you answer "who has rented Unit 2 since I bought this place" years later. If you file everything under tenant names, that history scatters the moment someone moves out.
Naming units so future you can read them
Use the name the tenant would recognize and the name that appears on the lease. "Main Floor" beats "Unit A" if that is what everyone calls it. Stay consistent across properties, because you will eventually see all your units in one list.
Avoid names that encode the tenant. "Sarah's place" stops being true, and then it is worse than useless.
In UnitDocs
Click into a property, click Add Unit, give it a name, and save. Leases, transactions, events and documents can all attach to that unit from then on.
For a single-family rental, add one unit anyway. The consistency costs you thirty seconds and saves the awkward special case later.