Any shooting position is allowed in a field target match. As long as you can see the hit zone and have the gun supported only by your body, you can use any position that you are comfortable with. Most shooters use a sitting position since that offers the best stability. However, some lanes may be set up to force you to use one position over another, ie. There are various holds that field target shooters use.
Most shooters rest their right elbow on their right knee.
The gun is then either rested on the left knee or on the left arm which is on the left knee . These are the most stable holds. Another hold is the military type hold military type hold where the left elbow rests on the left knee and the gun is held in the normal fashion. The high magnification allows the shooter to use the adjustable objective to rangefind. Range finding is the act of determining the distance to the target.