Coat of Arms
A coat of arms is a heraldic design, built around a shield, that represents a person, family, organization or state. Many national flags carry a coat of arms as a central charge, including those of Spain, Mexico and Paraguay.
Coats of arms are detailed by design, which makes them hard to read on a flag seen from a distance. NAVA's fourth principle is "No Lettering or Seals" for that reason. See what makes a good flag. Heraldry still gave flags much of their vocabulary, including field, charge and fimbriation.
If you want a heraldic feel without the detail, the Symbols tab in the flag maker has shields and crowns you can place and recolor. Keep it to one bold shape. The how to design a flag guide explains why simple symbols work better.