Both the "Due on Due Days" and "Due on Due Date" options are used for one-time assignments.
The information below should help you decide when to best use them:
Due on Due Date
Due on Due date is for use when you are doing a one-time assignment that you need to give to a group of users that needs to be complete by a specific date. For example, if everyone in a division needs to take a specific computer security training and it must be done by October 1, then you would use Due on Due Date and give it a Due Date of 10/1/23.
Due on Due Days
Due on Due Days is when you want to assign someone a task that must be done within a certain time frame of when they come into scope. For example, if you want everyone who is added to a division to have to take a one-time training on blood borne pathogens within 30 days of being added to the division, you would use Due on Due Days.
When using Due on Due Days, the number entered here should be the same as Show Days Before Due Date.
If you're using new hire functionality, use due on X days after anniversary instead of due on due days so that the course due date is X days after the user's hire date instead of X days after they are added to the system.