Stadiametric rangefinders allow you to estimate distance to your target without for example a laser rangefinder while limiting human error in case of estimating by using map.
The science behind it is simple geometry, feel free to look it up on Wikipedia.
Also included are some scopes from AT weaponry. Snipers are not the only ones using this.
As far as Prophet usage goes, I'd recommend getting used to mildot scopes as they can help you acquire approximate distances fast, helping with communication with your spotter. The PSO-1M2 is a sniper scope, but rarely used by Prophet.
NOTE: Works for default FoV of Arma 3. If you changed your FoV through config file edits, best of luck.
EXAMPLES OF STADIAMETRIC RANGEFINDERS
NOTE: You can open these images for higher resolution version (1440x1440).
Also is true for PSO-1M2-1 which is the variant for 9x39 mm - i.e. VSS Vintorez.
In the following image you'll see the reticle of PSO-1M2 mounted on an SVDM.
1) A vertical line and number 1.7. You want to place the line at the bottom of the target. The 1.7 means the rangefinder is accurate for target that is 1.7 meters tall.
2) A curved dashed line with numbers 2, 4, 6, 8 and 10. The top of the target is supposed to touch the line. In this example the soldier is placed in a way he fits. The numbers are meters in hundreds (200, 400, ...) of distance.
Based on these two points you can see the target here is approximately 550 meters far. From there you can adjust your scope (PSO-1M2 uses vanilla zeroing) and fire.
Side note: the three chevrons under main aiming chevron are used for engaging targets that are further than 1000 meters. Each chevron is additional 100 meters.
Mildot scopes are very simple to use - if you know how.
The dots on the scope's reticle are spread apart by 1 milliradian (also known as mil or mrad) is an SI derived unit - it's one thousandth (0.001) of a radian.
1 mil equals to 10 cm at 100 meters, 20 cm at 200 meters, ..., 100 cm at 1000 meters.
1 mil in scopes is the distance between centers of two dots on the reticle (see image below).
In the image below we're looking at a man who takes roughly 2 mils of space (very slightly more). The man is at ease and Arma's character height is 1.8 meters (less when in combat ready stance).
Given these parameters we can estimate the distance to the man is ~910 meters (900 for 2 mils and extra 10 as an estimate for taking slightly less space due to posture and angle). Laser Rangefinder confirmed this value (although the exact match was kind of a luck).
A simple formula if you prefer it this way:
( [Height of the Target in cm] / [Amount of mils] ) * 10 with result in meters
Again, it the image below, it would be (180/2)*10, which is 90*10 which is 900 meters. 180 is the target height, 2 is amount of mils the target takes and 10 is constant when using centimetres.
Also worth mentioning are AT reticles - while they are not typical choice for Prophet teams, I might as well include them.
You might notice a similarity with the PSO-1M2.
1) A vertical line and number 2.4. You want to place the line at the bottom of the target. The 2.4 means the rangefinder is accurate for a target that is 2.4 meters tall. Coincidentally that's an M1 Abrams.
2) A curved dashed line with numbers 2, 4, 6, 8 and 10. The top of the target is supposed to touch the line. In this example the tank is placed in a way he fits. The numbers are meters in hundreds (200, 400, ...) of distance.
Based on these two points you can see the target here is approximately 200 meters far. From there you just take aim with correct values (in this care we have tandem round loaded and we'd use the middle numbers - the bottom of the reticle) and fire.
Somewhat simpler is the base-game MAAWS sight - it's rangefinder is to the right of the reticle.
1) A vertical line. You want to place the line at the bottom of the target.
2) Several dashes with numbers 200, 400, 600, 800 and 1000. The top of the target is supposed to touch the dashes (or estimate). In this example the tank is placed in a way he fits.
Based on these two points you can see the target here is approximately 600 meters far. From there you just take aim and fire.