Jim clarke's chemguide website credible
Is UCdavis website credible?
http://chemwiki.ucdavis.edu/Physical_Chemistry/Quantum_Mechanics/10%3A_Multi-electron_Atoms/Quantum_Numbers
He termed 'principal shell'
Cs toh study guide book termed 'quantum shell'
https://www.dropbox.com/s/v8jixw73omiuvoe/20151029_221750.jpg?dl=0
At higher levels, things get more complex and specialized, and so
chemists most often give each other some face, cut each other some
slack, and allow some degree of flexibility in terminology usage
(eg. physical vs inorganic vs organic chemists often use different
terms differently, similarly for different branches of mathematics,
physics, biology, engineering, medicine, etc) as long as the
important main ideas are successfully communicated.
As such, Cambridge will probably leniently* ignore your use of the
term "quantum shell" or "principal shell", but you won't catch
Cambridge using these erroneous terms themselves in their Mark
Schemes.
Edited : *in an earlier post I said Cambridge was reasonable,
not lenient. This applies for critical Marking points and contexts.
But in contexts such as "quantum shell" or "principal shell", or
non-chemical spelling mistakes, which are non-critical and won't
affect the main important points required by the Mark Scheme,
Cambridge can be lenient.