I wanted to compare the gamuts of the two printing modes "sRGB" and PD, and to the sRGB's gamut. I don't have any utility software that directly draws the gamuts from the ICC profiles, and I had to use the soft-proofing capability of my usual editing software, and particularly the out-of gamut alarm, which shows which colors from one colorspace fall outside of another colorspace..
I made a small chart, which represent the colorwheel, where the saturation varies from 0% at the center to 100% on the edges. The three colorwheels represent the shadows, the midtones, and the highlights:
Note that in these colorwheels L is not constant (the L of the Lab space), and that the edges do not represent the edges of the Lab space, but rather the edges of the colorspace attached to the file. This is not as accurature as gamut drawing in a suited utility, but at least this chart can give qualitative informations.
Comparison sRGB versus Frontier native gamut (PD mode)
I have attached the sRGB profile to the file, and I have soft-proofed it using the PD mode profile. The out-of-gamut alarm is activated, and the sRGB colors which fall outside of the Frontier native gamut are grayed.
In the midtones, the native gamut is significantly smaller than sRGB, particularly in the green and blue tints.
In the highlights, the two spaces tend to be closer together, with still a slight weakness in the greens. The reds, magentas, yellows and cyans are perfectly reproduced.
In the shadows, the native gamut entirely contains sRGB
Comparison Frontier native gamut (PD mode) versus Frontier "sRGB" mode gamut
Now, the PD mode profile is attached to the file, and the soft-proofing is done towards the "sRGB" mode profile. Obviously, the "sRGB" mode gamut cannot contain the whole sRGB space, given that the native gamut does not contain it either. However, the question is to know if the "sRGB" mode gamut is the same or not than the PD mode one (all the native gamut used or not).
We observe that some colors of the native gamut fall outside of the "sRGB" mode gamut: mainly the saturated cyans in the midtones. In other words, the "sRGB" mode does not use the saturated cyans of the native gamut (these cyans also fall outside of the true sRGB space -see previous paragraph).
Conclusions
It is difficult to draw conclusions with 100% certainties, all the more than we know nothing about the internal color management algorithms of the Frontier. However, we can reasonnably assume that in the "sRGB" mode the files are converted to the native space with a rendering intent other than "perceptual": such a conversion would fill the entire native gamut, and what we observe is that the "sRGB" mode gamut is something like the common part of the sRGB gamut and of the native gamut. I tested a bit with the soft-proofing, and none of the usual rendering intents give exactly the Frontier "sRGB" mode result. My best guess is that it is something between "preserve identical colors/relative colorimetric" and "preserve saturations/graphics".