Creating custom pages - How to use them properly?

Hello,

i am new to osTicket, so this might not be a clever question: What is the use of creating a custom page?

I would like to know how i could use PHP-code on that custom page. Where is a custom page saved? It has to be somewhere in the database i think...
Is there a tutorial or something for using custom pages? Unfortunately i could not find a "how to" or something similiar.

Thank you very much in advance!