Customer Removable Units (CRUs) are components of a computer system that is external to the computer tower, and are considered replaceable by the user of the system. Generally customer removable units will not cause a partial or total system outage if replacement procedures are not followed correctly.
Customer removable units on desktop computers include the monitor, keyboard, external cables, mouse, AC adapter, and external peripherals such as a printer, speakers, external modem, any external back-up device, external CD-ROM drive and external Zip drives. On a notebook computer, CRUs are typically any part which can easily be removed without unscrewing anything from the notebook itself. This could be the battery pack, CD-ROM drive, DVD drive, and sometimes, the hard drive.
CRUs are generally considered to be non-reimbursable by the manufacturer. An individual manufacturer will be able to provide you with a CRU list for your computer, as the list may vary based on the exact model of computer or notebook, and may vary from one manufacturer to another.