The caps of all Difrax baby bottles feature volume markers. On the Natural S-bottle and the easy-grip bottle, this is 30ml; for the Wide S-bottles, this is 50ml. Because of the removable base on the Difrax baby bottles, the volume marker on the bottles only starts from 50ml. The volume you can measure with the cap compensates for the missing markers on the S-bottle itself.
This is useful in the early days when you are still having to measure out very small quantities, or need to know exactly how much milk is left after over a feed. The volume markers on the cap are calibrated, allowing for very precise measurements.