Riddle Me This . . . What Do These Greek Letters Mean?

[MysticCat]

I stopped at a stoplight behind another car yesterday and saw a decal on the rear window with these letters, all in white:

YFCPXXX

The decal was immediately beneath another decal, also all white, for USA Hockey; they might have been part of the same decal, although I can't imagine why they would be.

Any ideas on what these letters mean? (Or is it obvious to everyone but me?)

[FHwku]

they rhyme. maybe it's a double super secret hush hush handshake code, like in the movie Road Trip.

[Senusret I]

Or a phone number....