Hello David!
Expression is nothing more than an additional volume fader. In most cases I would leave this fader in the same position. For making volume changes during a project, you can automate it with CC12, if you have loaded one of the VI presets for Sibelius.
Best regards,
Andi
that's right, though it's only relatively recently that I fully understood this. Volume is designed as a main volume and Expression responds to the marked dynamics in Sibelius. Or rather I should say that CC11 responds to the marked dynamics in Sibelius and it is up to the user to decide whether to allocate Expression or Velocity XF to CC11. Indeed you can do both but that can lead to some pretty weird effects and I (in common with others I have read) don't reccommend this approach.
So --which to use? They are most certainly not the same in the way they scale dynamics and i'm not sure how often this point has been made clearly. Using the VSL House Style and CC11 for Expression, the dynamic range is too great leading to pp or quieter being almost inaudible. Reducing dynamic range can help but I haven't found an entirely satisfactory setting. On the other hand, if you allocate CC11 to Velocity Crossfade, the dynamic range is far less --indeed I would boost it to around 115 for a string quartet (but not all the way to the top). I assume that this house style assumes control through VelX, otherwise the dictionary will have to be slightly customised which is exactly what I've been doing for years to reign in the extremes but now think it's perhaps the wrong approach. The Expression slider should then have a fixed volume around the 90+ mark becuase even though Sibelius dynamic markings no longer control it, it still acts as a volume control. As you say, you can allocate a channel to Expression to allow direct control through a Sibelius command but this shouldn't often be necessary.
In addition to using the VelX for CC11, you then have to decide whether to activate Velocity X fade which is completely different but confusingly (well it certainly confused me for ages) has effectively the same abbreviation. This, as previously pointed out, changes the nature of the sound quite considerably and is best suited to more volatile writing --either fast and a bit wild or vibrato-rich and "romantic" for slow passages. It simply asks to be tried out and you can put a ~C28 (or whatever channel you allocate), 127 or 0 in the score to change between them as desired.
David