How can I update a field in the AR_RECEIVABLE_APPLICATIONS flexfield in Receipt lines via API?

Summary:

Hello,

We need to update a DFF field at Receipt Line level (the lines that represent each application), but I can't find any documentation mentioning which web service would be needed and how its request would look.

We've previously used the ErpObjectDFFUpdateService web service for filling flexfields at header level in Receipts, but we need to know how to do the same at Line level.

The following document deals with filling receipt dff's, but only at header level, it doesn't mention how to specify receipt lines in the request:

Doc. Title: How to Update Receipt DFF - Descriptive Flexfields using updateDffEntityDetails operation? (Doc ID 2563101.1)