How does " @n + @m " work? (missing doc explanation)

The doc https://fityk.nieto.pl/data.html#dataset-transformations says
" returns @n with added y values from interpolated @m"
How exactly @m will be interpolated relative to @n ? I.e, how should X values of @m be relative to Xs of @n for y values to be added?

From experiments, I see that when @n and @m have same X values, then new @ has same Xs and y vals are added.

Once I know, I can edit the doc myself.