ARD Window of Opportunity

[SPNHKATE]

This is another issue that has been discussed many times...but I need to ask one more question about it.  I know that you can adjust the ARD and must do so before the assessment window is over. My problem is in understanding what "assessment window" means.  Does it mean the MDS cannot be changed after the very last date allowed for the ARD including grace days (ex. we have 1-5 days for the ARD for a 5-day MDS) or does it means including grace days??  Can it also include up to the 14th day allowed for the R2b date (completion date)??   Thanks again for your help.