hello @martindunne,
the sequence start thing allow you to use sequencer 1&2 as list of values (if you set its bpm to zero to make it more convenient), you can scan it from whatever source you like.
that's the main use of this for me, but one can imagine some other uses..
So, if you use keyboard as matrix source, you can get sequencer values by segment of keyboard :
for example, with a linear keyboard source, keys 0 to 8 (c-0 to g-0) would get value from step 1,
keys 9 to 16 (g#-0 to e-1) would get value from step 2, and so on...
these example patch show you how to use this : have a different sound by groups of 8 keys ( = 128 midi notes / 16 steps).
you can use this to make drum kits or any varying sound across the keyboard range.