As you can see the picture below, if you change the rotation value by rotating the parent object (box), the rotation value of the child object (monkey) is retained. Child objects have their own local coordinate system, depending on the parent axis of the coordinates.
But today I found something strange when I tested the action constraint. To control the action, I have targeted a bone connected to the IK bone and set the action constraint's space option to local.
At first I thought this setting would not work properly. Because the value of the target bone is always connected to IK bone and has a rotation value of zero, like child objects that depend on the parent object. But unexpectedly, this setting worked just like the GIF below.
When I test the root bone by rotating it, the local space works well.
Why does the action constraint's local space option work correctly when the rotation value of the target bone doesn't change? I want to solve this problem and understand Blender's coordinate space clearly.
Here, I also attach the blend file I used for the test.
Thank you in advance.