Tenancy Manager purposely uses the term "letting unit" rather than property (or similar) because it's a more accurate reflection of how the system works.
A letting unit can be anything which you let on a tenancy (or some cases licence) including:
room in a shared house (where the room is to be on an individual tenancy)
car parking space
commercial premises (such as shop, cafe etc.)
Tenancy Manager can handle HMO type lettings in one of two ways:
if you let the whole house on a single tenancy, you will only need one letting unit for the whole house. When adding a new tenancy (see later) you will be able to add up to 8 individuals onto a single tenancy agreement
alternatively, you might grant tenancies on a per bedroom basis. In this case, each room would be an individual letting unit.