I've just started using Platinum after being a loyal, curmudgeonly SONAR 7 user for many years. There is apparently no offline help for Platinum, and the online help says that there is a phase reverse button on each track - and that product help is for X2.
 
How do I reverse the track phase in Platinum?

It's shown on page 496 of the Reference Guide. It's a circle with a diagonal line through it in either Track Inspector or Console view.

The Reference Guide pdf and chm local help files for Platinum are in the folder with the program, SONARPLT.exe. Make sure the SONAR Local Documentation package is installed.
 
X2, X3 and the current SONAR documentation are available here. Note: the current help is above the X2 and X3 listings
The current help for Track Inspectors may be found here
The phase button is in the top row point to by E in the image below, second from the left.

As an electronics tech, I associate Polarity as + or - in Voltage.  e.g +9VDC or -9VDC.   I associate "phase" the way Sonar interprets it. e.g. "180 degrees out of phase" which is phase inversion.

As firebird said, polarity, as taught in electronics classes, means voltage, not phasing.  This is the way I was taught in college.

^^^ Correct. "Polarity inversion" is the correct term to use when referring to microphone and speaker wiring, but "phase inversion" is the proper term for electronics and digital audio. That, btw, is why the the Greek letter phi (ϕ), the electrical and mathematical symbol for phase, is also the near-universal icon for this switch on both hardware and software mixers.
 
The confusion, I think, stems from the way the switch has traditionally been used to correct improperly-wired microphone cables or to invert their signals (e.g. top and bottom snare mikes). In this case, we're using phase inversion to effectively implement polarity inversion.

Caa2

This is why I referred to the button by location in the Track Inspector instead of the image used on the button face.
 
Edit: restored an image of Caa2 original post. For some reason this user likes to erase their posts.

Because there are no real "language police" in the audio industry, I've seen both terms used (then again this is an industry that for a long time couldn't figure out which pin of an XLR had to be "hot"). Some people say "phase" isn't correct because phase relates to frequency, and flicking a switch doesn't have an inherent relationship to frequency even though it does reverse phase. Others say "polarity" is correct because positive-going signals become negative-going, and vice-versa.
 
Most people refer to this function as a "phase" switch, then someone corrects them and says it should be "polarity," and then everyone says "yeah whatever, but when you say phase people know what you're talking about." And that seems to be about the extent of the standarization.

bitflipper
That, btw, is why the the Greek letter phi (ϕ), the electrical and mathematical symbol for phase, is also the near-universal icon for this switch on both hardware and software mixers.

So the real problem is that SONAR's phase inversion switch icon looks nothing like a Phi; the slash should really be vertical. No wonder the OP couldn't find it.