camera controls – What does the Custom White-Balance *icon* represent?

I already know what White Balance is (at least enough to get by!) I already know what Custom White Balance is.

All of the pre-configured White Balance options on the Canon DSLRs have meaningful icons – pictures of the sun, clouds, tungsten and fluoro lights, etc.

But the Custom White Balance icon is a mystery to me. It is a round-corners black rectangle upon two white triangles. (There’s a picture here from this site.). Update, here is the pic:

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My idle thought after some late-night experimentation with white-balance is: What are these shapes supposed to represent?

(Update: Originally I thought this was just Canon, but answers below indicate other brands use it too, including Pentax and Olympus. I checked quickly before originally posting, and Nikon have a different name for it (Preset), and no icon in the manual I checked.)