This is the key used by Business Object Modules to uniquely identify an instance of a Business Object of a particular type. Where the Business Object supports versioning, the key should identify a particular version of a Business Object. It must be unique within a Business Object Type, and it must be possible for a Business Object Module to uniquely identify a Business Object in persistent storage using the key. To identify the Business Object Key for a Business Object, the implementation can choose to provide a combined value representing the primary key attributes of the Initial Entity. The Initial Entity is the root entity of a Business Object Type. For further information, please refer to Analyzing the Folder Business Object Type.
The primary key attribute of the Folder Business Object Type's Initial Entity is folderID. So, the value of the folderID attribute can be used as the Business Object Key for Folder Business Objects. Similarly, the primary key attribute of the User Business Object Type's Initial Entity User is userID. So, the value of this attribute can be used as the Business Object Key for User Business Objects.
Note that it is also possible for an Initial Entity to have multiple primary key attributes. For example, take the example of a Locale Business Object Type with Initial Entity Locale. The Locale Entity includes the attributes language, country, and variant. To uniquely identify a particular Locale, the combined value of the attributes language, country and variant are required. So, the Business Object Key for the Locale Business Object must contain the values of these three attributes.