I found this phrase in a theory about directional equivalence and I'm not sure what it means and if it is part of linguistic aspect of translation.
Formal-aesthetic equivalence is part of the five frames of reference that defines Koller's Bezugsrahmen der Übersetzungsäquivalenz, which are:
The formal-aesthetic equivalence type is summarised as
the ways in which the ST author uses language to convey their meaning
The most obvious examples of this aspect of translation arise in translating literature, and very strongly related to the issue of "personal voice / style".
Part of Koller's framework is that priorities have to established when translating, and not all of these can, need to, or should be satisfied.
Analyses of translation quality make use of such frameworks, and Koller's is one of the more influential.