Possible return values are listed in ESOUIDocumentationP5.txt
* GetItemType(*integer* _bagId_, *integer* _slotIndex_)
** _Returns:_ *[ItemType|#ItemType]* _itemType_
h5. ItemType
There's no direct way to get the name of the constant equal to the value returned by GetItemType, you'd have to build a table yourself or use LibConstantMapper.
The name is only useful for debugging, or if you want to use it in saved variables rather than integer. In code, you don't need to care what the integer value is.