What's the FM algorithm in Prologue?

I’m trying to figure out where Osc 2 sits in the FM algorithm in Prologue. Here’s the relevant manual entry on the subject, but I’m convinced there’s a typo in here because I can’t figure out when Osc2 is the modulator and when it is the carrier. The offending sentence is highlighted.

From the manual…
Frequency modulation or FM means that the frequency of one oscillator, called the carrier, is modulated by the frequency of another oscillator, called the modulator.
• In Prologue, Osc 1 is the modulator, and Osc 2 and 3 are carriers.
However, Osc 2 can be both carrier and modulator as if frequency modulation is applied to Osc 2 it is modulated by Osc 3. If Osc 2 also uses frequency modulation, Osc 3 is modulated by both Osc 1 and Osc 2.
• The pure sound of frequency modulation is output through the modulator oscillators.
This means that you should turn off the Osc 1 output when using frequency modulation.
• The Freq Mod button activates/deactivates frequency modulation.
• The Ratio parameter determines the amount of frequency modulation.

I’m going to see if I can grok it on my own now. Wish me luck.