Back printing is the term used when copy is printed on the back side of the envelope
Design features are the same as on the front of the envelope.
If the envelope is to be printed on the flap and can be done with the flap out than the flap can be printed at the same time as the front of the envelope. This is not back printing because the flap is not down and the underside of the flap is not being printed.
When the envelope is printed before it is converted, much of the back printing on the envelope is printed on the same side as the front of the envelope and when the envelope is folded, the copy becomes the back of the envelope.
Die-Cut Printed Paper before folding into an envelope