As with most things in computing, with CD-ROMs there is a standard (ISO9660), but also other formats one of which was another defacto standard.
When computing started using the then-new CD format for data storage in the early 1980s, a standard was needed for organising the data. In 1985 a number of leading computer companies came together to create the High Sierra format (named after the hotel where the companies' representatives met).
High Sierra became the defacto CD-ROM data format until the finalised ISO9660 standard a couple of years later. However, software support for the standard lagged it's introduction for a while which meant the High Sierra format continued to be used. As the format was supported even after ISO9660 replaced it, companies like Microsoft and IBM continued to publish CD-ROMs in High Sierra format well into the 1990s. This article is an interesting look at the format.
Not actually having an active computer with a CD-ROM anymore i don't know if my copy of OS/2 Warp is High Sierra or not, though as the follow-on version apparently was still in that format there is a good chance it is.
