Use the Optional parent option in the setup of a process reference field to limit the forms shown to users in certain situations and increase ease of data entry and integrity.
Consider the following example and its processes.
I have a "Vehicles" process to keep track of vehicles and their information. Note the staff reference field "Assigned To" on the vehicle form.
I also have a "Vehicle Inspection" process, to obviously keep track of vehicle inspection data. Using the staff reference field "Name" as the Optional parent for the "Vehicle Unit Number" process reference field, only the vehicles that reference a particular user will be shown. If I had not chosen the "Name" field as the Optional parent for the "Vehicle Unit Number" process reference field, the user would have been presented with all entire list of vehicles.
Below is the setup of the "Vehicle Unit Number" process reference field, note the Optional parent "Name" which is a staff reference field.