Using Alternate Dispatch Groups To Find The Lowest Common Denominator
When you click the Create Field Order button on the Group Field Activities transaction and leave the Override Dispatch Group blank the system attempts to find a "lowest common denominator" dispatch group and assign it to the pending field activities. If it is successful, all of the field activities are grouped under a single field order and dispatched. If it's unsuccessful, a separate field order is created for each activity without a common dispatch group.
The following example will help illustrate how a lowest common denominator dispatch group is assigned to a group of field activities.
FA1: Install Meter
FA2: Read Meter
FA3: Test Meter
Initial dispatch group assigned to field activity
NorthWest - Electric - Install and Remove
NorthWest - Electric - Readers
North - Electric - Testers
Alternate dispatch group
NorthWest - Electric
NorthWest - Electric
North - Electric
The above table illustrates the following example:
There are 3 field activities at a premise and each of which has a different initial dispatch group (see the first row in the table).
Each initial dispatch group has an alternate dispatch group.
Of these field activities, FA1 and FA2 have a common alternate dispatch group: NorthWest - Electric. FA3, however doesn't have a common dispatch group. This means 2 field orders will be created:
The first field order (associated with the NorthWest - Electric dispatch group) will contain FA1 and FA2.
The second field order (associated with the North - Electric - Testers dispatch group) will contain FA3.
Fastpath: 
Please refer to Automatically dispatching field activities for another example of how the lowest common denominator is selected.