From the Hero Games website:
"All the characters and spells from Arcane Adversaries in Hero Designer format."
Hero Designer is a piece of software that greatly simplifies the creation of HERO System content by providing templates, error-checking, and a full set of powers, skills, etc., and more.
Hero fans (usually uncredited) volunteer to turn content from HERO System titles into Hero Designer format. No artwork is included in HD Character Packs.