With all due respect, these statements are wrong.

Here is how it works by the book:
a) The amount coming from a DBC voucher is being converted into a penalty, even if you purchase a full Y.
b) For a residual value there will be a MCO issued stating that it's a DBC residual value with the very same expiration date as the original DBC voucher. The remark that it's a residual DBC value is supposed to give the ticketing agent the hint that travel has to commence by the expiration date of the voucher.