Containment drawings show how cables are physically routed through a building — tray, trunking, and conduit runs carrying power and data between distribution points. This is a tight coordination point, since containment competes for space with ductwork and pipework.
Containment routes that aren't properly checked against other services are one of the most frequent sources of site clashes. Drafting this carefully up front is far cheaper than re-routing cable tray after it's been installed.