Operating System Command actually doesn't have a well defined multiplexing format. That 'first number' is not in the standards. It's a thing that has haphazardly grown as people have tacked more and more stuff onto OSC. The standards don't even guarantee numbers in the command string at all.

In fact, you can say the exact same thing about OSC (it was meant for one protocol, the operating system's) as you just did about APC. Because that's what ECMA-48 actually says: 'The interpretation of the command string depends on the relevant operating system.'

There is no operating system standard for this in the Unices or in Linux-based operating systems. As I said, this is something that haphazardly grew in the last decade or so as people decided that they needed some kind of terminal control string, and how dtterm and xterm had been processing OSC (very limited compared to all of the things that people have nowadays tacked on) was there.