Answer: Here is how our sort works:
1 - Sort works by Date which means the Last Modified Date - no caveats. We commonly say Last Modified Date for documents and Send Date for emails but that is only because we typically expect the LMD for an email to be the date it was sent. This is not always true.
2 - Family - sorts by the Family Date which is keyed off the LMD of the parent.
3 - You can map documents that come in without a date. The system populates a date of 12/31/9999. These entries actually float to the top of the sort.