English is funny. So many different ways to talk about the same thing.
In compressed air for a plant a "loop" drawing could be a Piping and Instrumentation Diagram (P & ID), or other schematic drawing showing pipe sizes, where it comes from, and what equipment it connects to. Sometimes the air is supplied through a "Ring Main", which becomes a complete loop, allowing the air to flow to any of the equipment from both directions of the loop. The loop is a continuous run of main supply piping which runs around the plant and connects back to where it started.
Another drawing is an "isometric" which is a dimensioned drawing for fabrication of the pipe spools.
Another drawing is a General Arrangement which shows in Plan, Elevation and Section, the route of the compressed air piping, the connected equipment, and the main dimensions for the design and detailing of the piping.