Yes that exact term is not used at the moment by the Brg teams of Marriott/SPG. However that term is useful for Marriott if there are multiple flexible rates available and the customer reserve the highest flexible rate and try to do a LFN claim against that rate. That wouldn't work.
At the moment you need to book the lowest rate for what type of room category, flexible/non-flexible cancellation and amenities you are comparing against. It's very often that there are multiple different rates for the exact same of room category, flex/non-flex and breakfast/non-breakfast.

Not to create any bad luck or something but I'm sure in 1-2 years Marriott will force us to book the absolute lowest rate no matter above and compare only that. That would create a Hyatt-disaster and basically ruin the Brg program.