Table HRUS_D2 significance to SAP DMS

Hello Guru,

Can anyone try to explain to me how table HRUS_D2 been use in SAP DMS and SAP Easy DMS for user substitute for another user?

we have problem recently that a document which has 3 reviewers, (A,B and C). when the document is set for review only one reviewer been notified sample only A. reviewer B and C did not receive email even though they are also indintified in DMS as reviewer.

we found out that only A was notifed since in table HRUS_D2, A was been a substiture for B and C. Please explain to me the usage of table HRUS_D2 in DMS. Thanks.