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Consider a motorcycle. Nothing new and fancy with foo foo. The classic. There is no automated clutching, no automated shifting, no programmed shifting logic. A rider will do all the clutching and work his way through the gears manually selecting each sequentially one by one in most cases. But not all. He might be on the brakes hard on his way into a turn and downshift twice but not engage the clutch on the first shift. He hasn't skipped a gear but he has skipped engaging the clutch in an intermediate gear.
This describes a fundamental aspect of a sequential transmission. It isn't called a sequential for nothing, depending on the transmission type you may be able to skip engaging the clutch on a gear to bypass it but you cannot skip the gear itself. Fellows complained about this aspect of the Xtrac. He could not skip gears shifting down because the transmission has automated shifting but no shift logic. Every downshift includes engaging the clutch on that gear.
With that groundwork you can then look at some of the available DCTs out there. Mine is like a motorcycle, it can skip a gear or more by not engaging the clutch but it must shift through each gear sequentially. It is both a dual clutch transmission and sequential. Not all dual clutch transmissions are however. Some can skip a gear or more without going through the intermediate gear(s) on the way which is not sequential shifting.