It's a fairly common way to do advancement in large companies, and is usually based on experience. "Software Engineer I" is entry level, "Software Engineer II" has 3 – 5 years of experience, etc.
Can you fill in "etc."? I'd be interested to know what definitions they used for the levels (and whether it was consistent across different sources of data).
Beyond II (intermediate dev, not a college hire, the majority of working engineers) there is no meaning beyond "suitably impressive resume or reputation at the employer".
