From the design:

forward_to_shards

A boolean. Default false. True for some fields in the structs defined in command_generic_argument.idl, see Specify generic arguments in IDL. If true, this parameter should be included in commands that are forwarded from mongos to shards. (This field replaces today's stripFromRequest concept, with a flipped meaning and improved name.)

forward_from_shards

A boolean. Default false. True for some fields in the structs defined in command_generic_argument.idl, see Specify generic arguments in IDL. If true, this field should be included in replies that mongos forwards from shards to clients. (This field replaces today's stripFromReply concept, with a flipped meaning and improved name.)