Imagine you have two animations now, one for standing shooting, and one for running; Do you want the character to run and shoot at the same time; Do you need to create a new animation? No need, you can easily use a part of each animation（standing shooting and running） and combine it into a new animation by using it（Avatar Mask and Animation Layers ）;

The active poriton page of your Puppet Stage reminds me of Unity Engine's Avatar Mask and Animation Layers.
【1】Make each normal animation file editable for Avatar Mask;
【2】Then two animation clips can be added at the same time on the timeline;
[1]The Avatar Masks of the upper and lower animations each control a part of the body, which can achieve the shooting while running as I mentioned above.
[2]If the Avatar Masks of the upper and lower animations are the same, then let the one above cover the one below;
And you can easily switch between the upper and lower animations with just one click. The advantage is that when you are editing, you hesitate which animation to use, and you can use this method to quickly preview the animation

https://docs.unity3d.com/6000.1/Documentation/Manual/class-AvatarMask.html
https://docs.unity.cn/cn/2020.3/Manual/AnimationLayers.html