I believe it is numeric from 1 to MAX_NUM_BATTLEFIELDS or some other nonsense. The tooltip on the minimap button uses it to build the text..
Code:
for i=1, MAX_BATTLEFIELD_QUEUES do
status,mapName,instanceID = GetBattlefieldStatus(i)
if instanceID ~= 0 then
mapName = mapName .. " " .. instanceID
end
end