There are different systems for guitar multi-effects, which depend on their intended use. Two common concepts that have established themselves are rack-mounted and floorboard versions. For a while, desktop units like the Vox Tonelab were also available, but they have become less common. Many multi-effects units can now be edited via a computer app or a tablet, making it unnecessary to have them within arm's reach on your desk.
Compact pedals are the smallest multi-effects units, thanks to powerful processors that allow a wide range of effects to fit into a small space. It starts with dual pedals, which combine two effects in one enclosure, controlled by knobs. These could be two different overdrives, as with the Strymon Sunset, or delay and reverb, like the Keeley Caverns. Slightly larger compact pedals, like the Strymon Mobius, focus on one effect category, such as modulation effects like chorus, flanger, phaser, etc. Another modern classic is the Eventide H9, which offers all effect types in a relatively compact format. These devices deliver high-quality effects, but the downside is that you can only load and use one effect algorithm from their arsenal at a time. For those who need multiple effects simultaneously in a compact form, options like the Line 6 HX Stomp, Boss GT-1000Core, or smaller models from Zoom or Mooer might be more suitable.
A drawback of compact multi-effects is the limited control options, which is inherent due to their small size. In compact pedals, you typically move step by step through individual preset spaces, whereas floorboard units have more switches, providing greater flexibility on stage.
You can call up a preset (a preconfigured combination of effects) to which you can then add individual effects, which is often programmable. Some floorboards have a built-in expression pedal, which allows you to adjust effect parameters in real-time or authentically control effects like wah-wah or whammy. Floorboards, as the name suggests, sit on the floor in front of the guitarist and are operated by foot. They usually feature a display for navigation and show preset data and effect parameter settings. These settings can be edited using various knobs, which can be somewhat cumbersome when the device is on the floor. Of course, there's nothing stopping you from lifting it onto a table for this purpose, but most floorboards can now be edited comfortably on a computer app or tablet screen.
The 19" format is still practical for securely housing an effects unit in a studio rack or for live performances. This format typically involves two units: the actual effect unit in the rack with all the connections, where you make all the settings, and a floor unit (foot controller) with switches and a display for remote control. The effect unit and the floor unit are connected with a cable. This approach is beneficial for live performances because you can place the rack with all the connections and possibly a receiver unit for your guitar transmitter somewhere out of sight, and only the floor unit is placed on stage. This minimizes clutter on the stage floor.