Can I set the expression pedal to control different parameters in different patches or is it a global relationship - i.e Delay Mix?
The expression pedal assignment is per preset. You can choose and control one different parameter per preset. So it can be i.e. Delay Mix in one preset, and Chorus Depth in the next one, etc.
However, if you decide to control the Volume with the pedal, it will be for all presets, as Volume is a global parameter only.