There are three types of job assignments: recurring assignments, job slot assignments and non-slot assignments.
Non-slot assignments are not associated with job slots and may have some or all of the above fields above filled in. VSys will let you leave out information that you find unimportant or unavailable from these assignments, and you can enter fields with conflicting information if you're not careful.
Recurring assignments are assignments repeated on some schedule into the future. These are commonly used for long-term volunteers who do the same job repeatedly, "until further notice".
A job slot assignment is associated with a job slot, and gets its job, job group, location, sport, start date/time and duration from that job slot, so all of those fields are filled in and cannot be changed.