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What is this metal plate for?
You can also see it here:
Is found on many aircraft so what is that piece of metal right there?
@Greg is correct. It is a scuff plate, to protect the fuselage from damage due to GSE (like jetbridge etc). The image below shows it.
Image from slideshare.net
The image below shows an example made out of Ti from a manufacturer's site.
Image from exoticmetals.com
Required, but never shown