If the order of execution is left totally unspecified (random selection)
Then attainment of specifications requires "weak fairness"
The order of execution of UC's is chosen at random with the constraint that each UC will be executed infinitely often during the total execution of the computation
This is the end point of "no" specification of order
Intermediate points include "optimistic" protocols
UC initiates execution without certain knowledge that a given execution is in a minimum schedule - validates sequence before emitting outputs.
Example - Database transaction processing.