Document units allow you to set a physical measurement unit for your document that suits your way of working, whether imperial, metric or in pixels.
As you create a new document, its document units will be set depending on the preset chosen. For example, presets for A-series page sizes will use millimeters as default, while US page sizes and photo page sizes use inches; for web-related page sizes, pixels are used.
You'll encounter document units when you're:
At any time, you can change your document units. The setting is saved with the document.