When you schedule a job you can check the technician is available and has the right skillset for the job (based on whether you have assigned that Job type to their Staff profile).
When selecting technicians for a job, they are represented in the drop down list by colours (as follows). You can place the mouse cursor over the name to see an explanation to why they are unavailable.
Green - available to do the job and have the skills for the selected job type
Red - they are booked to another job (details in the yellow box)
Orange - they do not have the right skills and should not be assigned to do the selected job type
Dark grey means they are marked as unavailable for any job at that time
See manage technicians and schedulers profiles to find out how to allocate job types to technicians and Mark staff as unavailable