What is ⌘ Cmd + Space keyboard shortcut for?

Firefox Developer Tools (macOS) - Open autocomplete popup  

RStudio (Mac) - Attempt completion  

Reserved Mac - Show or hide the Spotlight search field (when multiple languages are installed, may rotate through enabled script systems).  

Reserved Mac - Rotate through enabled script systems.  

Apple macOS - Show or hide the Spotlight search field. To perform a Spotlight search from a Finder window, press Command + Option + Space. If you use multiple input sources to type in different languages, these shortcuts change input sources instead of showing Spotlight  

elementary OS - App launcher  

Firefox (macOS) - Select or deselect the tab with the dotted rectangle. The other tabs stay selected or deselected.  

iPad Magic Keyboard - Show / Hide Search field  

Xcode - Show or hide the Spotlight search field (when multiple languages are installed, may rotate through enabled script systems)  

Corel Painter (Mac) - Zoom tool (zoom-in)  

Adobe Photoshop (macOS) - Switch to Zoom In tool  

PureRef (macOS) - Focus Canvas  

Luna (DAW) - Toggle Record  

Luna (DAW) - Toggle Record  

Luna (DAW) - Toggles recording. Press Command + Space to start recording. If you press Command + Spacebar while LUNA is recording, the session continues to play, but the transport drops out of record.  

Luna (DAW) - Restarts playback from the original playhead position or the start of the selection.  

iPad Smart Keyboard - Spotlight Search  

iPadOS 13 - Show or hide Search field  

Smartsheet (Mac) - Selects the entire column of your active or selected cells  

Nuendo 10 (macOS) - Toggle selection  

WireShark 3.4.3 (macOS) - Reset colorized conversations  

Adobe Acrobat 2017 (Mac) - Change whether the row with focus is in the selection