This measuring tool is a guide to estimate the diameter of nipples, particularly the tip of the nipple and not the base (certainly not the areola) in order to determine the correct sizes of shields and flanges for breast pumps. Note that the measurement for nipple shields is different than for a breast pump flange, moreover, the nipples can be of different sizes on the right and left side.
When it comes to breastpump flanges, it is important that ONLY the nipple is moving in the tunnel of the flange, it can touch the walls without worry (unless, of course, if discomfort or pain is felt). In the past years, the indications were suggested differently (it was recommended to add 2-3mm), but with this new method of measurement, the results are really more effective! Here is the link to learn more about this revolutionary method: Jeanette Mesite Frem's Flange Fits Guide
When you have the right size flange, you should feel something moving, but no pinching. In addition, the milk should flow more easily (ideally in sprays).
When the flange is too large, it might take some of the areola which could cause injury and/or minimize the volume of milk expressed, thus compromising milk production.
Take into consideration that it is better to measure the nipples at rest. The measurement between the nipple at rest (unstimulated) and the one that is stimulated after a few minutes of breastfeeding or pumping can be very different, the nipples tend to be very reactive and swell a lot.
To estimate the size of a breast shield, we must take into consideration not only the diameter of the nipple, but the length of the tip which can vary according to the oral tolerance of the baby (hard/soft palate).
This tool is a guide and cannot replace the advice of a health professional (knowledgeable and competent of course)