Often there are more customers than the available vehicle fleet can
serve. Then it would be nice to assign priorities to customers that must
be served to differentiate them from those that can be served later or
omitted at all.
One can now assign priorities just by adding
"priority": 1 to services and shipments. There are 3 priority levels: 1 = high priority, 2 = normal (and default) and 3 = low priority. It is documented here.