When you create a new root Business Object in the migFx Target Map, the Studio will automatically create an Interface Field called PartitionId on the new Business Object.
This field is special and is marked by a different (light-blue) background color than the rest of the fields. While you can modify this field (set Key and change Name, Format and Description), you cannot delete it.
migFx requires all root Business Objects to have a Partition Field (which is why you cannot delete it). When data is actually being migrated in the migFx Runtime environment, migFx will use this field to partition the data by the value in this field.
It is then possible to decide which users have access to view the data for a given partition. While members of the migration team typically will have access to view the data for all partitions, it is possible to limit the access of external users (user from the business being migrated).
This comes in particularly handy if a migration project is migrating more than one business. For instance if several Banks are migrating from one Banking System Provider to another in one migration project. Using this partition mechanism, migFx will enable you to control that users involved in the migration from one bank will not be able to see the migrated data from the other bank(s).