The doubtful receipt entry is posted on the customer account if this field is set to No, or on the "doubtful customer" control account if the field is set to Yes.
This information has an impact on the account types that will be used at the time of posting the doubtful receipt entry.
See the documentation on the accounting accounts for further information.
The behavior for the matching is different depending on whether the company manages the VAT on collections (DCLVATPAY parameter) and whether the customer is considered as doubtful.