Adding a parameter for milestone classification, so an older build will not override a newer build if only they belong to the same group.

The parameter would group different milestones. The ordinal parameter is used to order the milestones in the group. The abort behavior in a group is the same as now.
extended simplified script from above:
sleep PARAMETER
milestone label: 'mileStone', ordinalGroup: projectName
sleep 5

Example1:
project1: started with PARAMETER = 20 (sleeps 20 sec), ordinalGroup: 'p1'
project2: started after project1 with PARAMETER = 5, ordinalGroup: 'p2'
--> project1 and project2 run in the same job

Example2:
project1: started with PARAMETER = 20 (sleeps 20 sec), ordinalGroup: 'p1'
project2: started after project1 with PARAMETER = 5, ordinalGroup: 'p2'
project1: started after project2 with PARAMETER = 7 (sleeps 7 sec), ordinalGroup: 'p1'
-> first p1 is aborted through 2nd p1 build, p2 runs through.