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Almost universally, in English electrical schematics, the letter "K" represent a relay. Thus K2 is "relay # 2". I don't have those schematics in front of me but in my past experience looking at Amel's electrical diagrams they really aren't schematics showing all the electrical componentry, and aren't really wiring diagrams either. They are more on the order of what I would call "block diagrams", that show the major components & general theory/layout without very many of the details.
Thus the control circuitry for the K2 relay probably isn't depicted but somehow the idea that the relay is controlled by something else might be depicted.
Hope that helps,
s/v Liahona SM #335