These are unique numeric identifiers for instance maps. They can be obtained by calling the GetInstanceInfo API function, or from the tables below. These values can be used as a locale-independent way to identify instances. InstanceMapIDs are used in the default UI by GetChallengeModeMapTimes(InstanceMapID). The continents themselves are treated as instances and have unique IDs as well.
InstanceMapIDs are not interchangeable with MapIDs despite many similarities. The map name returned by GetInstanceInfo() is not necessarily the same as the name returned by GetRealZoneText() and GetMapNameByID(). When denoted below, Zone Name is the normal map display name, and Instance Name is the name returned by GetInstanceInfo().
To help fill in this list, go into an instance and use the command
The number shown in slot  of the output is the InstanceMapID, which can be added to the table below.
/run local id = GetCurrentMapAreaID(); local n,_,_,_,_,_,_,iid = GetInstanceInfo(); print(iid, GetZoneText().."/"..GetRealZoneText(), n, id, GetMapNameByID(id))
Which is: ID, Zone Name, Instance Name, MapID, Map Name