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 the Difrax baby bottles have a removable base, 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 need to measure out very small quantities or want to know exactly how much milk is left after a feed. The volume markers on the cap are calibrated, allowing for very precise measurements.