Our planning strategy for BUS involves choosing to perform the rewrite only if the planner selects a compatible index. But because the query planner can only choose the index based off match predicates (we don't push down any sort information past the unpack buckets stage). But, the planner always chooses a forward (collection, index) scan to satisfy match predicates. This "soft" hint will tell the planner if you're trying to match a predicate on say (meta, time) it would be nice if the scan happened in this particular order as opposed to the default order. This is intended as an internal-only feature of the planner.