Together with BatchDrawCommand.flags, this specifies how to depth sort the instances in this draw command.
If BatchDrawCommandFlags.HasSortingPosition is set, this contains the index of the first position in the BatchCullingOutputDrawCommands.instanceSortingPositions array which contains world-space positions that Unity uses to depth sort the instances. In this case, set this value to the index of the x component of the world space position of the first instance.
If BatchDrawCommandFlags.HasSortingPosition is not set, Unity casts this value to a float and uses it as an approximate depth to depth sort the instances in the draw command.