This option lets you specify the direction in which you'll be travelling along the motorway. For example, on the M1 you may travel in either the northbound or southbound direction.
We take this into account when working out the distance of hotels from the motorway, because in some cases you can't leave the motorway at a specific junction, for example. We base the distance from the motorway of a given hotel on actual driving directions, so if you'll need to continue along the motorway and turn around at the next junction, or if you must take some other route to reach your chosen hotel, then we take this into account automatically.
You can always leave this as the default option of "Either", but choosing the right option will help us calculate more accurate results.