This is a coding example working on IRIS 2020.1 and on Caché 2018.1.3
It will not be kept in sync with new versions
It is also NOT serviced by InterSystems Support !
In most cases, a global used by default storage has just 1 subscript level that represents the IDKEY.
For an index-globals we may see 2 or more subscript levels.
Arrays, or parent-child relationships or persistent classes extending a base data class
are examples where we see more levels. Though all these globals are quite uniform.