A reference to the prescription to use for the work.
The prescription format is assumed to allow multiple products, and to be split into multiple prescriptions by a plugin if needed.
A reference to the machine configuration / implement configuration / triggers / meters to use for the work. This is better than a reference to Implement that was originally here because it allows for implements that perform multiple operations; e.g., BluJet AT4610 that performs liquid fertilizer injection and tillage at the same time.
Note that this is optional because when the WorkItemOperation is created, we may know what we want to do, but not the machine or implement we want to do it with.
Added for clarity; analogous to WorkItem's inclusion of MachineId