Reconciliation - Alternative / Special Reconcillation Account

Hello,

Can someone through light on this.

Q1: What are the scenarios we use Alternative Recon A/c and Special Reconciliation Account?. To my knowledge we use SRA's in terms of down payments, guarantees... my question is around what makes SRA different from Alternative Account.

Q2: In SPRO, while configuring the above two accounts i see we are assigning these accounts to primary reconciliation a/c. Why we are doing this? and do postings the will be made to both Primary RA and ARA or SRA?

Appreciate your time.

Thanks