These numbers could be given to you as a continuous string:
The grid of the map can be used to define the route by the crossing of gridlines.
This example is simply a list of the numbers of the gridlines across which your route must travel in order.
66 66 22 21 67 21 21 68
These numbers could be given to you as a continuous string:
6666222167212168
or with a couple of spot heights mixed in to confuse you:
666622128216721219768
Devious organisers will use this method when you're in an area of the map where the northing and easting gridline numbers are similar!
This route could also be given as the North, South, East or West sides by which you should LEAVE each grid square:
W E S S E N S E
or even as the direction from which you will ENTER each grid square:
E W N N W S N W
Note that these directions will only be specified using the four cardinal compass points, N, S, E and W. You will not get a NE or a WSW direction in the string.
On another variation of the theme, this same route may be defined as Top, Bottom, Left and Right departure from grid squares:
L R B B R T B R
Each grid square can be specified by a four figure map reference, defined by the intersecting gridlines at the SW corner of the square, so you may get this list of grid squares through which your route must pass in order:
6622 6522 6622 6621 6620 6720 6721 6720 6820