With the content container the editor defines filled structure elements as reusable units, which he can include user-defined. The clue is: the included elements are instances of the original unit. These instances can be changed individually or on several pages simultaneously. If several instances are changed, a new unit is generated.

In practice this means: you are writing i.e. a contact address into a structure element and call it contact. Now you can integrate this contact address via the element box into any number of pages.

If the contact changes for a particular unit of your homepage, i.e. in your product catalogue, you edit the text on one page and then you can choose on which page the change should occur.
The text is being changed on all selected pages, on the pages not selected the original text remains.