Void = Creates a negative invoice transaction to reverse the original transaction. You cannot Void an Advance Bill.
Write Off = Typically used if the client is not going to pay for the invoice. This will create a credit memo and apply the "Open Amount" on the invoice and post to your Bad Debt account.
Close = Used to recognize the unused portion of an Advance Bill and creates a new regular invoice to post the Unapplied Amount to your sales.
Create Credit = Converts the "Unapplied Amount" of the advance bill to be a credit memo, so it can be printed and sent to the client to use against future client invoices.
Post = This affects the General Ledger based on the Posting Date of the transaction.
Approve = Helps to separate workflow from accounting procedures.