RE: Anyone can explain me this syntax?

Thanks Wendell for your complete explanation. While reading your help, I tested another alternative:

ROW[. = key('relacion_x_cobertura', REL_ID)[1]]

and it worked too.
...
I think that case is equivalent to generate-id method since all ROW's has at least 1 field different, making the whole ROW different. Is that true?

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