0.9.10+
A composite key reports multiple keycodes at once.
Let's say there is a five-keys pad. You can make the most useful (?) programming tool like this:
kbd.add_layer :default, %i(KC_SPACE CUT COPY PASTE KC_ENTER)
kbd.define_composite_key :CUT, %i(KC_LCTL KC_X)
kbd.define_composite_key :COPY, %i(KC_LCTL KC_C)
kbd.define_composite_key :PASTE, %i(KC_LCTL KC_V)
Instead, you can also write the equivalent keymap in this way:
kbd.add_layer :default, [ :KC_SPACE, %i(KC_LCTL KC_X), %i(KC_LCTL KC_C), %i(KC_LCTL KC_V), :KC_ENTER ]