The serialization format for sort keys changed between 4.2 and 4.4. In order to ensure that this did not disrupt the upgrade or downgrade process between 4.2 and 4.4, we introduced a new use44SortKeys flag on the aggregate command, as well as a similar _use44SortKeys flag on the find command.

These flags are no longer necessary in 4.6, since a 4.6 node does not need to remain compatible with 4.2. Most of the relevant code related to the 4.2/4.4 upgrade will be deleted under related ticket SERVER-43361. However, a 4.6 node still needs to be able to accept find and agg commands with the use44SortKeys flag. During upgrade, a 4.4 mongos or mongod might send this flag to a 4.6 mongod. Therefore, a 4.6 will tolerate the presence of the flag, but can completely ignore its value.

In 4.7 development, we can finally remove all remaining traces of use44SortKeys, since a 4.8 node should never receive a command with this flag set.