This took a bit of digging, but it looks like this is meant to be most useful in conjunction with the RetentionStartDate property of an item.
In the VAC, you can choose to have a Retention Category start counting from the Modified Date or the Archived Date, but if you're manipulating an individual item with the Content Management API (either inserting or updating), you can also set a RetentionStartDate from which that item's "retention clock" starts ticking. I think the idea is that a partner using the API could set a zero-day Retention Category and also set an item's RetentionStartDate to a day in the future, and therefore can specify the exact date on which the item shall expire. This mirrors the behavior of the "Fixed expiry date" option that's in the Retention Category Properties, except that it can be assigned to an individual item.
This was one of a complement of Retention and Records Management features introduced in EV 12.2 that cater to U.S. government customers operating under National Archives and Records Authority (NARA) regulations.
--Chris