Recommendation ITU-T M.1400 covers the designations of interconnections among operators' networks of circuits, groups, group and line links, digital blocks, digital paths, data transmission systems, digital blocks created between digital circuit multiplication equipment (DCME), virtual containers and multiplex sections.

The designation information is in two layers:

Layer 1: The unique information; the designation;

Layer 2: Additional information; the related information.

Guidance for the user is provided in a series of examples.