This allows you to make certain employees "assignable" for jobs so they can be put on schedules. Employees will not be able to be placed on certain jobs/trucks, or show up to be placed on jobs/trucks until they've been granted permission to be assigned under HR scheduling.
To do this you need to go to "Manage > HR > Employees" to bring up a list of the employees then select the employee you want to place on jobs.
Click on "Scheduling" then select "Yes/No" based on whether you wanted them to be assignable or not.
Once "yes" has been selected so that a specific employee can added, when you go to the "Daily Schedule" their name should be selectable so you can add and remove them from jobs at will. (shown below)