Michelle,
AC is Accomadation Certificate. II randomly gives them out when you deposit a unit they deem nice to have in inventory. It allows you to obtain a studio($199), 1bd($249), or 2bd($299) unit at select resorts based on a grid they provide. Most often they are for off/non prime weeks at resorts in areas that aren't in high demand. Its not a lot of fun having to search through their inventory 6 selections at a time as they don't allow you to do an ongoing search. Their website isn't exactly designed to make the search easy.
We got one last year for depositing a Door County unit that we used to book for Disney this year. Unfortunately or not, we did a lot of vacationing last year and just couldn't find a use for it and they expire 1 year after issue.
As soon as we got this one, we went looking and found a week over Turkey day up in Eagle River. We will be doing a lot of traveling/flying this Spring and Summer and wanted something in driving distance. Branson is one of the areas available, but no weeks were available during our kids breaks from school.
Bill