Hi Frederik,
Generally speaking, IEntry is a financial entry, whereas IDebtorEntry/ICreditorEntry are debtor/creditor entries, respectively.
Internally in e-conomic, it's really the same register. Thus, at least for now, every IDebtorEntry and ICreditorEntry will always have exactly one corresponding IEntry - but not necessarily the other way around.
For example, when booking an invoice of DKK 10.000 + 25% VAT, the following kinds of entries will be created:
1. IEntry on the sales account, -10.000
2. IEntry on the VAT account, -2.500
3. IEntry on the customer group control account, +12.500
4. IDebtorEntry on the customer, +12.500
(3) and (4) are internally the same, but exposed separately in the API, as described above.
However, we split it in the API since it seems to be the more logical data model people are used to from many other accounting systems.
If an IEntry's type is DebtorInvoice, then VoucherNumber will ALWAYS be an invoice number - i.e., one you can find via the IInvoice interface. For all other entry types, no matching invoice will exist (or possibly make sense

).
I'm kind of lost as to what it is you're trying to accomplish - detached from e-conomic-specific terminology.
What event triggers you to want to find exactly what information?
Best regards,