As already stated, it does depend on the type of aircraft you fly. Although it can also varry between mod states for the same aircaft type!
A good example is for the 757. There are two types of FMS CDU: Pegasus and non Pegasus. If you get an INSUFFICIENT FUEL message in the sctatch pad of a non Pegasus CDU, you must press CLR before you can enter any new data in the scratch pad. For a Pegasus CDU, you can write over the top of the message and then line select. After line selecting, the message will reappear. Alot less annoying!
