Hi Tur Muji,
Interim Table is generated in the First step of Archival i.e. Generate Candidates. It creates a table which has the Primary Key of the Parent table/Driving Table being archived with other three columns accompanying it. This Primary Key column holds the all those Primary keys of the records which are inactive and can be archived. Basically these primary keys will be fetched on the basis of the where clause provided in the Entity, all those records which falls true as per that where clause will be considered as inactive and it will be populated in the Interim Table with the Purgeable Flag set to Y i.e. it means these records are inactive and can be archived.
Staging tables are exact replica of the Source tables which are being archived with all the columns exactly as the Source tables. It contains the records whose primary keys are present in the interim table. this will be populated using the insert statements in the Entity.