From DoA BJD Wiki
Option parts are one way to customize ball jointed dolls. Option parts are made by the dollmaker or an independent artist to replace default pieces of a doll. Many companies offer optional hands in different poses, or even jointed hands; some offer different feet that allow the doll to wear high heels or sandals. Some sculpts that use a faceplate system may have alternative faceplates (usually with different expressions or a sleeping face, for example) to swap on the doll.
In recent years there has been a rise in fantasy-theme optional parts, such as horns, hoofed legs, resin wings, and mermaid tails.