The Shimano XTR BR-M900 Cantilever brake arms come in two different lengths. Which go in front, the long ones with the stamped "F" or the shorter ones?
I presume you have two sets of two, and the picture shows one of each?
In that case the pair marked F would be front-brake. The longer brake arm gives you more mechanical advantage on the wheel where it helps the most.
If you have a single pair and they're mismatched in length, then something's wrong and two pairs have been mixed up.