Forum Post: Applies To Doc No

Can anyone tell me how this field gets populated on the Cust or Vendor ledger entries?  I've created a CM and, in the header, entered in Applies To Doc Type of Invoice and then an Applies to Doc No.  Once that CM gets posted, that field does not get updated on the ledger entry.  It has the Applies to Doc Type but, no Doc No.

Is it something that was used way back when and just not used anymore?  I am running on 2013.

Thanks