In accounting, the amount of accounts that can be generated with customers, vendors and other types of entities can be very large.
For this reason, SAP has the option for creating a reconciliation account, that functions like this:
When you create a posting, this posting is directed to an account created for a vendor or a customer, an in its master data, the customer or vendor has assigned a reconciliation account.
So when you post a debit or a credit amount to this vendor or customer, you are also affecting the G/L Account.
In order to post to other reconciliation accounts, you can use SGL.