About Availability of Parts
Part availability is a constraint only for the Optimizer, not the ABS. Data loaded into the Optimizer cache includes all trunk inventories for all field service engineers in a service region.
The Optimizer - Consider Parts parameter uses the part availability in the trunk inventory for an engineer when considering the engineer for assignment to an activity. The Optimizer - Parts Period parameter determines the number of hours that the loaded part list is valid for an optimization. For more information, see Defining Cost Functions for the Optimizer and Parameters for Schedules.
If the Optimizer considers part availability, and if the activity includes a part with an order line item number, then you can drop-ship the part for the service call. The Optimizer does not consider the part as a constraint in scheduling.