How do scheduled payments interact with other processes?

Scheduled Payments works in conjunction with other automatic features during ledger entry and ledger import as follows:

The scheduled payments split is performed after the calculation of automatic tax and stoppages (only relevant in Ledger Entry). This is so that any tax is included in the debtor or creditor line before the transaction is split.
The scheduled payments split is performed before the standard Payment Terms calculation, so that document dates, discount dates and interest dates can be calculated on each individual instalment line generated in the split.

If a transaction is subject to a split via scheduled payments, it is only split once even if two or more processes affect the same transaction, such as holding the journal and subsequently posting it to the Actuals ledger. The split is only generated in Ledger Entry if the transaction has not been split already.