The team blog of the Expression Blend and Design
Steve White, Technical Writer, EID
Microsoft Expression Interactive Designer (EID) is largely about creating and editing trees of user-interface elements. When you use the double-click gesture in the Library palette (View > Library) to create a new element, Interactive Designer needs to know into which container in the tree to put the new element. So we have this idea of the 'active' or 'activated' container which refers to an existing container which will become the parent of the new element. Only one container in the tree can be activated at any time. By the way, I use the term 'activated' because the container in question is in a particular state, ready to receive the new element. 'Active' implies the container is somehow going to do something whereas in fact its state is passive. Either term is ok to use and they refer to the same idea.
So, how do you set a container to be activated and how do you identify the currently activated container? I would recommend that you go to the element tree (in the Timeline palette - View > Timeline) and expand the tree until you find the container you want to activate. Then double-click the container and note that it now appears in both the element tree and on the artboard with a yellow highlighted border. It's the highlighted border which shows which container is activated.
Now, when you double-click an element type from the Library palette (View > Library), EID will create the new element and nest it as a child of the activated container. If the activated container is one which may have only one child, and that container already has a child, then your double-click-to-create gesture will replace the previous child (and all its children, if any). This is sometimes what you want, other times it may be an indication that the activated container was not the one you intended. Always check for the yellow highlighted border before double-clicking from the Library palette.
The other way to create a new element is to select the element type in the Library palette and then draw it onto the artboard. EID will place the new element as a child of the container most appropriate to the location in which you are drawing.