Job associations connect people to jobs as Active, Inactive, Leave of Absence, Substitute or Other, Pending Preference, Resigned, Retired, Supervisor, Terminated, Transferred, Waitlisted and do this without making specific job assignments.
So how are job associations different from job assignments? A job assignment means you are expecting a volunteer at a specific time and place to do a task. Job associations are more fluid. Use job associations for your volunteers who work a flexible schedule. Rather than making job assignments for a volunteer who works a different schedule every week, give them an association. They are "connected" to the job, and can use that association to sign in on the kiosk. This would be a good way to track for example, a Pastoral Care volunteer, who comes in on an "as needed" basis to work with your organization. Their hours would still be recorded to the job that he did, but you are saved the work of trying to keep his schedule current.
Another common use is for substitutes: a volunteer might do one job on a regular schedule, but be a substitute for another job.