At bit depth of 24 bits, you have a theoretical dynamic range of 144dB. The actual dynamic range is less than that, though, due to electronic limitations. If you assume a real-world 24 bit dynamic range of 110dB (as specified for my Echo Audiofire 8), then tracking at, for example, -14dBFS peak still leaves you 96dB dynamic range - the full theoretical dynamic range of a CD. Tracking at such levels is about leaving headroom for processing, mixing, etc and keeping stuff away from digital clipping.
As I understand things, setting levels for tracking should be done in the analogue realm, ahead of the AD converter. If you use the levels controls in the software, you could simply be attenuating an already digitally clipped signal. Someone correct me please, if that is incorrect.
The signal does not have to be clipping in analogue to be digitally clipping. A perfectly clean analogue signal can simply be too hot for the AD converters analogue inputs which will cause digital clipping.
+1 for the Massive Mastering article.
Paul