The trapping sequence (also called the trapping order) matches the order in which inks are printed at the press, but it doesn’t match the order in which separations are produced at the output device.
The trapping sequence is particularly important when you’re printing with multiple opaque colors, such as metallic inks. Opaque inks with lower sequence numbers are spread under opaque inks with higher sequence numbers. This process prevents the last applied ink from being spread, and it still creates good traps.