Thereunto is a less common way of saying thereto, meaning to the thing that was just mentioned, as in Please fill out your personal profile and answer all questions pertaining thereunto (translation: Please fill out your personal profile and answer all questions pertaining to it).

What does the word thereunto meaning?

adverb. to that, unto that. moreover, in addition to that.

What does thereto mean in a sentence?

: to that a text and the notes thereto.