This is a piece of wargear available to Ordos Xenos inquisitors. However, that means it now features in 2 codices, C:Grey Knights and C:Inquisition. In C:I, it has the following item description:

ULUMEATHI PLASMA SYPHON
Models firing plasma weapons (as defined in the Warhammer 40,000 rulebook) while
within 12" of a model with an Ulumeathi Plasma Syphon will count as Ballistic Skill 1.
Now, how I read this is that it means that any model firing a plasma gun, plasma pistol or plasma cannon within 12" of the bearer counts as BS1, as these are the weapons in the "plasma weapons" section of the rulebook.
However, in the C:GK FAQ, an Ulumeathi Plasma Syphon affects the following weapons:

Q: What counts as a plasma weapon for the Ulumeathi Plasma
Syphon? (p62)
A: All Plasma weapons, as well as Eldar missile launchers firing
plasma missiles, burst cannons, starcannons, all Tau pulse
weapons and any weapon described as using ‘plasma’ as its
effect or in its special rules.
Now, this affects a far wider range of weapons than the C:I version, and is much more useful. Both cost the same number of points. The C:GK version also seems much more fluffy, and also actually worth the points.

If I included an Ordo Xenos inquisitor with an ulumeathi plasma syphon in my army, which rules would take precedent? If it depends on which codex you've selected the Inquisitor from, then does that mean I could have a GK main army, with an Inquisitorial detachment, and have 2 Ordos Xenos inquisitors with the same piece of wargear creating different game effects? If this is going to be updated, I seriously hope it goes the way of the GK version.