My question is to seek confirmation that the second opening is merely supposed as a measuring aid/help. An answer would clarify (i.e. from personal experience) that Starsan (as with the other sized bottles, is a "one component" sanitizer only and not as the second highlighted container suggests a "two component" thing.
Yes. Starsan is a single-component sanitiser and the small chamber is intended for measuring the liquid. You can't see it on your photo, but the chamber is marked with graduations in fluid ounces showing the quantity of liquid.
To use it, you remove the cap on the chamber then squeeze the bottle until the chamber fills to the desired level, then stop squeezing. The chamber doesn't drain back into the main part of the bottle, so the measured quantity can be poured into a measured amount of cold water.
I'm in the UK, so prefer to use millilitres; also the high dilution rate of Starsan means that a whole chamberful makes more sanitiser than I normally need. So I use a measuring syringe instead of the marks on the chamber. The recommended dilution rate is 1.5mL per litre.