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Grid referencing systems, frequently used in military applications, allow users to locate a specified, blocked section of a topographical map sectioned off into grids. These systems vary in accuracy; essentially, the more digits in a system, the greater its accuracy. The military six-digit variety offers accuracy of 100 meters. This means you can locate a section that measures 100 meters square.
Break down the six digits into two sets of three. The first set represents your "eastings," or grid lines measured off to the right. The second set represents your "northings," or grid lines measured up to the top of the map. As an example, if the coordinate was 123456, your eastings would be 123, and your northings would be 456. Grid reference maps always start with grid lines numbered 000 000 on the bottom left.
Follow the grid numbers to the right until you find the number corresponding to the first two digits of your eastings. In the example, you would look to the right for the grid line numbered 12.
Look up the grid line corresponding to the first two digits of your northings. In the example, you would move up the map until you found the northern grid line 45.
The point discovered with the four digits already used puts you at the bottom left of a grid section measured 1,000 meters square; this is the block you want. With the six-digit system, you can achieve greater accuracy within that block.
Divide the bottom of the block into 10 imaginary sections numbered zero to nine. Some maps provide these sections for you already. Follow these imaginary marks to the right until you find the one numbered the same as the last digit in your eastings. In the example, you would look for the number three.
Section off the left side of the block into 10 evenly divided, numbered sections, and find the one that corresponds to the last digit of your northings. In the example, you would look for line number six. You should now have located the bottom left corner of a smaller 100-meter-square block, referenced by the six-digit coordinate.