docs: specification of payment ledger

erpnext/accounts/README.md

@@ -10,4 +10,42 @@ Entries are:
 - Sales Invoice (Itemised)
 - Purchase Invoice (Itemised)
 
-All accounting entries are stored in the `General Ledger`
+All accounting entries are stored in the `General Ledger`
+
+## Payment Ledger
+Transactions on Receivable and Payable Account types will also be stored in `Payment Ledger`. This is so that payment reconciliation process only requires update on this ledger.
+
+### Key Fields
+| Field                | Description                      |
+|----------------------|----------------------------------|
+| `account_type`       | Receivable/Payable               |
+| `account`            | Accounting head                  |
+| `party`              | Party Name                       |
+| `voucher_no`         | Voucher No                       |
+| `against_voucher_no` | Linked voucher(secondary effect) |
+| `amount`             | can be +ve/-ve                   |
+
+### Design
+`debit` and `credit` have been replaced with `account_type` and `amount`. `against_voucher_no` is populated for all entries. So, outstanding amount can be calculated by summing up amount only using `against_voucher_no`.
+
+Ex:
+1. Consider an invoice for ₹100 and a partial payment of ₹80 against that invoice. Payment Ledger will have following entries.
+
+| voucher_no | against_voucher_no | amount |
+|------------|--------------------|--------|
+| SINV-01    | SINV-01            | 100    |
+| PAY-01     | SINV-01            | -80    |
+
+
+2. Reconcile a Credit Note against an invoice using a Journal Entry
+
+An invoice for ₹100 partially reconciled against a credit of ₹70 using a Journal Entry. Payment Ledger will have the following entries.
+
+| voucher_no | against_voucher_no | amount |
+|------------|--------------------|--------|
+| SINV-01    | SINV-01            | 100    |
+|            |                    |        |
+| CR-NOTE-01 | CR-NOTE-01         | -70    |
+|            |                    |        |
+| JE-01      | CR-NOTE-01         | +70    |
+| JE-01      | SINV-01            | -70    |