Both container and embedded element are elements. They are just playing different role according to the relationship they have.
If an element allows other elements to embed into its own region, then it can be used as a container and it can be called as container element. Once an element is embedded into a container, the element becomes an embedded element.
So far there are 8 types of elements can be used as container, they are:
You can right drag (drag with right mouse button hold) the current selected elements into container element. If your mouse does not have the right button, you can press the Control key on keyboard to simulate the right button. Below are some examples:
When you hold the right mouse button and drag other elements over the container, the region that can accept the elements will be highlighted with light blue color.
Once an element is embedded into the container, it becomes part of the container, however you can still select it by clicking on it, or select it from the outline view.
As you can see the embedded element will have a light blue border when it is selected, and its id is also in light blue color in the outline view.
To extract current selected elements from its container, just click the button on tools panel:
To learn more about conjoining elements, please read "Conjoin Multiple Elements" section.