The identifier type of the referenced object, represented as an integer.
{i} is the identifier type represented by one of the following integers:
0 = Null
1 = Imported Asset
2 = Scene Object
3 = Source Asset.
The identifierType constitutes the {i} element in the string representation of a GlobalObjectId, the format of which is:GlobalObjectId_V1-{i}-{a}-{l}-{p}