We all understand that it's often highly complex if not impossible for most devs to quantify an accomplishment. It's easy for some things to be quantified obviously - downloads, traction - though those aren't exclusive to engineering (could be marketing).

Scalability numbers and measurable performance improvements are relatively common. So if you built something that is able to handle n requests maybe.

The amount of revenue generated by a feature is clearly tough - but you might be able to find out new subscribers since the feature was introduced. Even if you can't, just saying you built a certain feature is probably enough in most cases.

It really boils down to unique, one-off accomplishments for me. Specificity. That code that runs millions of times - tell me the product name, what that product does, what your code does, and what tools you used to build it. Even if you can't point to a quantifiable figure, the specificity gives you some credibility.