I mean that one device is inside a suitcase/case and I would like to give the headline like user's manual but I would like to include words like device maintenance in suitcase/case or device operation in-case?
Possibly "Operating the device in/inside a/the case/suitcase" would do.
A "suitcase" is specifically a case that people normally use for transporting their luggage (clothes, personal possessions) when travelling. "case" is more general. Unless you specifically mean "suitcase" then probably you should use "case".
"a case/suitcase" makes it sound as if the device doesn't come with