Hello Marwan,
Thanks for your question. Below is an explanation of the two options:
With "Unapply and Void Check", the payment will be UNAPPLIED against the invoice vendor ledger entry, the invoice vendor ledger entry will become OPEN, and the payment will be reversed by the void check. Essentially, the vendor will incur a balance from the invoice, as if no check was applied against it.
With "Void Check", however, the invoice vendor ledger entry will still be CLOSED by the payment entry, and the voided check entry will still be OPEN. That means, the vendor will not have a balance outstanding for their invoice. The only impact you've done here is that you've voided a check, but not it's financial implications, neither in the subledger or in the G/L.
Let me know if this helps Marwan.
Regards.