Containment drawings show how cables are physically routed through a building — tray, trunking, and conduit runs that carry power and data between distribution points and end use. This is one of the tightest coordination points on any project, since containment competes for the same ceiling and riser space as 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.