In legal texts, at least in Sweden and Germany, it is common to print some parts of the body text in a smaller font. Is there a name for this typographic convention?
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I believe it is called an "Inline Citation" or "In-Text Citation"
Here: http://www.easybib.com/guides/citation-guides/chicago-turabian/notes/ I believe they could be referring to the "same thing" but lacking an actual citation (as in your example) simply as a "note".
Update: During some further research I found this document from Harvard: https://utas.libguides.com/ld.php?content_id=21757697 a solid 48 pages just concerning notes and citations. I stand by my original answer that this is simply an "in text note", or possibly a "parenthetical reference" (also known as Harvard referencing) but the document does provide fascinating (to text nerds!) further reading.
In Polish text Norm such things are called "Additions", "interjected terms" by publishers and "bracket definition" by lawyers. Or "Parenthesis" by linguistics.
The short definition of such text is
two-side isolated intra-wording sequence
And in proofreading marks they are symbolised by [p] and [w]