Replum is a thin false septum, present in fruits such as siliques and some legumes, replum separates two valves of fruits.

False septum in Mustard is also called a replum which grows during the process of dehiscence in fruits. It separates two valves of the fruit.

Question on this can be asked in the AIPMT, NEET,etc.

Previously the whole membrane dividing the fruit into two halves was called Replum. Now Replum only refers to a layer of cells between two valves on which ovules are borne and the rest of the membrane is called Septum.(See The Arabdopsis Book published by The American Sòciety of Plant Biologists and available free online.