In bold, stamps issued by the entity which it belongs or is most
connected to. This allows to separate “authoritative” (or, at
least, well informed) depictions from casual ones, and, on the other hand
(by counting the non-bold occurrences), also to gauge how much a flag is
popular in foreign stamps.
In italics, stamps showing one single flag, be it as the main
design element, or as a detail — thus those in roman style show more
than one flag.