When and where can I use them? Commercial use or personal use only? Referring to DIN Alternate and Condensed which come with OS X.
I tend to use typefaces I've DL'd so I know what the licence is, but dunno about system fonts?
I would add that system fonts can potentially change when you upgrade to the next version of Mac OS X. There's no technical reason not to use them in your work but personally I'd be nervous. Especially with Yosemite on its way. *cough* *nudge*