New Character. This option lets you select from generic men, women and children. The included characters give you full control of height, and hair, skin, top, bottom and shoe color. Built in characters require minimal resources and memory — you should default to using them wherever possible, especially for extras or minor characters.
Import Character. This option lets you import a Character from the ReadyPlayer.me Avatar system. It provides a lot of customization including clothes, hat, glasses and more. While these avatars are fully posable and you may set different expressions, you can’t adjust their colors. They also require up to 20mb per character.
Read a bit more about it here
Existing Cast Member. If you already have your cast created and customized, this will let you quickly choose from existing cast members when dropping a new actor into a shot.
Import 3rd Party. If you are working with a 3D artist or third party character creation tool, Previs Pro allows you to import those characters as well, you can read more about it here