Final answer:
A Personal Portable Oxygen Concentrator for airline use must be foldable and compact and able to fit under the seat or overhead bin. Warranties and aesthetics are not airline regulations.
Step-by-step explanation:
When a Personal Portable Oxygen Concentrator (PPOC) is determined to be permissible for use on board an aircraft, there are a few important criteria it must meet beyond the technical and safety specifications.
Firstly, the PPOC must be foldable and compact, allowing it to be easily transported by the passenger. Secondly, the device should be able to fit under the seat or in the overhead bin to adhere to airline space limitations. Although warranties and aesthetics of a PPOC are considerations for a consumer, they aren't critical when it comes to airline policies and regulations for on-board use.