How to order or set default option for Mass Update select2?

When selecting Mass Update from List View on, for example, Cases, the select2 for the field that is to be mass updated has a certain order.

Can I sort that list and/or at least choose what the default option is?

It seems from massupdate view that I should be able to set the defaultOption in metadata somewhere... but where/how?

thanks,

FrancescaS