This short Ableton tutorial covers what Racks are and how to create them by grouping devices in your session. Grouping MIDI effects produces a MIDI Effect Rack, grouping an instrument produces an Instrument Rack, and grouping audio effects produces an Audio Effect Rack.
The placement order inside an Instrument Rack follows a fixed signal flow: MIDI effects or MIDI Effect Racks go before the instrument, and audio effects or Audio Effect Racks go after it. Getting this right keeps your signal chain logical and your Detail View clean.