Because of this clear division of labor, there are useful features which are the sole responsibility of the MUA, even though it might seem that Supercite should provide them. For example, many people would like to be able to yank (and cite) only a portion of the original message. Since Supercite only modifies the text it finds in the reply buffer as set up by the MUA, it is the MUA's responsibility to do partial yanking. See Reply Buffer Initialization.
Another potentially useful thing would be for Supercite to set up the
outgoing mail headers with information it gleans from the reply buffer.
But by previously agreed upon convention, any text above the
mail-header-separator which separates mail headers from message
bodies cannot be modified by Supercite. Supercite, in fact, doesn't
know anything about the meaning of these headers, and never ventures
outside the designated region. See Hints to MUA Authors, for more