The Source menu lets you select the type of source to drive the current character. This menu is enabled in the Character Controls window, regardless of the whether the HumanIK tool is active or not.
The options in the Source menu are:
When Stance input is active, the current character is forced to its neutral character definition pose - the pose set during character definition using the properties in the Definition tab. You can toggle the character in and out of Stance input without affecting any other input source, and without unlocking the character definition.
Setting the character to Stance input is useful when animating multiple characters in the same scene, or for troubleshooting retargeting issues. When adding a prop object to be handled by your character, Stance input lets you set up the prop with your character in a neutral position, then switch the input source to test how the prop reacts when motion is applied.
Lets you use a Control rig as the source of mapping data to constrain the character.