Keyframes are set on the current take’s BaseAnimation layer when you do not set keyframes on multiple layers.

Duplicate the BaseAnimation layer’s animation to create another layer with the original animation, or modify the original animation with new layers and merge them later.

Animation Layer editor A. BaseAnimation layer

When your workflow involves setting keyframes on layers to modify the animation on the BaseAnimation layer, you can lock the BaseAnimation layer to prevent it from receiving keyframes as you set keyframes on other layers. See Muting, Soloing, and Locking animation layers and Setting keyframes on layers.

The animation on the BaseAnimation layer is always part of the result animation. Though the BaseAnimation layer can be locked, it cannot be muted, soloed, or parented to layers. See Muting the BaseAnimation and Result animation.

Unlike other animation layers in the Animation Layer editor, the BaseAnimation layer cannot be reordered or deleted. It is always at the bottom of the stack, with animation from the other layers added on top of it.