sequence number
The number or letter an object carries inside the group it belongs to, when several members share one name and one address.
A group of buildings put up as one development often gives each building a number instead of a name of its own. The Olympic Athletes' Village in Seoul has ninety-six buildings that share one name and one address, and only the number 101, 232 or 321 tells them apart.
The same applies at every level of the hierarchy. A complex carries a number inside its cluster, and a wing or a roof section carries one inside the building it belongs to.
The value is a number or a short letter code, as the developer or the register assigns it: 232, A, 5A, II. It is not unique beyond its own group, because every large estate has a building 101.
An object that carries a name of its own has no sequence number. The two are alternatives, not companions.