Please explain Default Assignment Rules to me!

Summary

Please explain the various options in the Default Assignment Rules tab when setting up courses

Content

Hi all

We have a number of courses that need to be completed on an annual basis, but I'm struggling with understanding what option to choose when setting up the default assignment rules. We want people to access the new assignment 30 days before the old one expires.

Previously, we had used:

Validity Period Options: After Completion

Expiration: Expires in Years

Every: One Year

Renewal Options: Period Before Expiration

Renewal Period: 30 Days Before Expiry

However this was not working correctly when we tested 20D a couple of weeks ago.  I was advised that I should be using the 'Validity Period Option' After Due Date.  But I'm confused as to why I'd use this and not After Completion?