MEP Elements all have connection ports: these are represented by hotspots to which you can connect another MEP element.
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:
Note: The regular ARCHICAD library includes MEP-compatible Library parts, such as plumbing fixtures, which have connection ports that connect smartly to MEP elements. To display these MEP connections, enable the Model View Option to Show Object MEP Connections. (Document > Model View > Set Model View, and go to the Miscellaneous Settings for Library Parts panel).
See also Behavior of MEP-Compatible Objects.