These were pointed at one another (through holes in the tube of the telescope, or by removing the telescope from its mount) so that the crosshairs in their foci coincided.
The player also has the ability to lock onto enemies by moving the crosshairs close enough to the target and clicking and holding the right mouse button.
The focal plane is thus located outside of the eyepiece and is hence accessible as a location where a graticule, or micrometer crosshairs may be placed.
Even crude grease pencil crosshairs were quickly discovered to enable a pilot flying in a pylon turn to hit a stationary area target with relative accuracy and ease.