The term "exergue" is a specialized numismatic term referring to the space on a coin or medal usually located on the reverse side below the main design. A possible synonym for exergue could be "subscript area".
The subscript area implies a space beneath something where additional information or details are placed. Just as an exergue is a specific area on a coin or medal for inscriptions or dates that are secondary to the main design, a subscript area also serves as a location for supplementary elements. For example, on a historical document, there might be a subscript area where a footnote or additional comment is placed. This is similar to how an exergue on a coin contains details that complement the main imagery.
Another possible synonym could be "lower margin space". This emphasizes the location of the exergue being at the lower part of the object and being a distinct space similar to a margin. For instance, in a printed layout, the lower margin space might be used for page numbers or small annotations. In the case of a coin or medal, the exergue in the lower margin space can hold important historical or commemorative information.