MEP Elements all have connection ports: these are represented by hotspots to which you can connect another MEP element. (Certain elements in the regular ARCHICAD library, such as plumbing fixtures, now include connection ports, allowing you to include them in MEP routes.)
A newly placed MEP element that is connected to an existing MEP element will take on the connection parameters (e.g. shape, size) of the element to which it is now connected.
Connected MEP elements can be moved and edited together.
The hotspots of correctly connected, straight MEP elements are black:
Unconnected hotspots are displayed as diamond-shaped editable hotspots:
