https://www.apple.com/shop/refurbished/about

Apple Certified Refurbished Products are pre-owned Apple products that undergo Apple's stringent refurbishment process prior to being offered for sale. While only some units are returned due to technical issues, every unit is evaluated to ensure it meets Apple's quality standards.

Each Apple Certified Refurbished Product:

• undergoes full functionality testing and any defective modules identified in testing are replaced.
• Apple Watch and Apple Pencil devices that require replacement parts are not included in the Apple Certified Refurbished Program.
• is put through a thorough cleaning process and inspection.
• is repackaged (including appropriate manuals, cables, new boxes).
• includes either the Operating System originally shipped with the unit or, in some cases, a more recent version.
• is placed into a Final QA inspection prior to being added to sellable refurbished stock.

Refurbishment procedures follow the same basic technical guidelines as Apple's Finished Goods testing procedures.