[edit: perhaps the key uses differential pulse-position modulation and assumes a reasonably constant insertion rate after some initial header?]
What that means is simply that the signal is serial and that the position of a bit in the train determines its position in the resulting data item.
In that sense, every serial protocol ever invented except for analog protocols that encode multiple bits per timeslot (more than one bit per Baud to confuse you completely) is pulse position modulated.
And even in those there is usually a fairly tight coupling between the ordering of the blocks of bits in the packet encoded, though technically there wouldn't have to be a sequential ordering, you could for instance interleave the bits from two different timeslots. Not that I'm aware of any crazy encoding like that but it could be done.