I bought a circular polarizing filter, to take some shots of people on a balcony with a city skyline behind them on a bright-but-cloudy day.
However, I've never used a CPL before and don't know what I'm doing — I'm s so far just rotating the filter randomly and trying it out. Despite that, it nevertheless does seem to really help darken the sky, make the texture of the cloud surfaces more visible, and create a better balance between the sky and the people standing on the balcony near my camera (with 2 off-camera flashguns to add some fill flash as well).
There is a ∆ (triangle) marking on the rotating ring, shown in the attached picture below. This marking really looks like it is trying to tell me something. (I mean something perhaps like: "For maximum polarization, rotate this marking to position it at the 12 o'clock position, if the sun is at position X and the lens direction is Y.")
The filter came with no instructions or manual. What does this marking mean, and how is it used in practice?
The filter is an 82mm Marumi EXUS Circular P.L filter, fitted onto a EF 24-70mm f/2.8L II USM lens. (The weird "P.L" punctuation is part of Marumi's product name.)