The bracket that the caliper is mounted in, near the window, has two purposes. One to store/mount the caliper with the cable connection to the computer and the other as a means to check the accuracy of your caliper. Ponsse calipers have no calibration to do. They are simple and easy to use but can become damaged over time. In order to check your caliper there are four holes in the caliper bracket that are marked 0 mm, 100 mm, 200 mm and 350 mm, as shown below right.
Checking from zero to 100 mm, 200 mm and 350 mm while reading the caliper will check it for accuracy. This also helps the operator to understand how much pressure he can put on the caliper as he is measuring. If the light touch of the caliper in the first holes measure 102 mm, for instance, then adding a little pressure will give you an accurate 100 mm. But if a light touch of the caliper is exactly 100 mm then you know not to add any pressure when measuring the trees during calibrations.