Add the 9, 11, and 13th. Theoretically they are piled up, as in examples bellow, but in practice some notes are omited when adding these extensions. Otherwise they become dissonant or even impossible to play(depending on instrument).
maj7 + 9
m9 + 11
11 + 13
Its about changing the order of the notes in sequence of a chord, its usefull for:
Arrange bass/treble lines, on a chord progression.
Different flavors of the same chord.
Get reasonable fingerings.