Set shifts occur over just one day, with a fixed start time and end time which is listed on the job page. Generally these jobs will contain enough information for you to arrive at the shift at the time given in the job information, and complete the shift.
Ongoing jobs are posted over a range of dates, and the specific start time, end times, and breaks are not always specified in the job.
These details will be communicated to you via the job description or in briefing information from the client when you are selected for the role.
Generally ongoing jobs will occur over multiple days and you will need to be available for all available shifts.