What is meant by 'Memo Voucher', 'Post dated - Voucher' and 'User - defined voucher' ?

Memo Voucher: It is a non-accounting voucher. It does not affect accounts of the user. The entries contained in these vouchers are recorded/stated in a separate register, but not as a part of ledger. Post-dated Voucher: Some software allows the users to enter the voucher for future transactions which are usually similar as the previous once. User defined Voucher : In accounting software there are $\displaystyle 23$ pre-defined vouchers. It allows the user to define or create new accounting or inventory vouchers as per the requirement.