Member-based organisations can have different durations for members. Some may have a fixed membership year eg. Jan 1st to December 31st each year, which all expire on the same date. Other organisations may work for a fixed period based on the joining or renewal dates. The duration can be defined to cater for each of these scenarios.
If your membership runs for a fixed length of time, based on the joining date, then select Specific Period and enter the number of days, weeks or years your membership will run. If your memberships run to a specific end date, select Specific End Date and enter the date your memberships will expire.