Is there any serious difference between logoff, logout, signoff and signout?
Websites seem to use the phrases interchangeably. Is this just a difference is style, or have they made a deliberate choice?
In the past when there were no computers, in office or in any secured place, people sign in by signing their signature and time in while entering into the place. When they leave, they sign out by putting the time out and sign. Website followed these words (sign-in and sign-out) in the similar way when computers came.
The notebook that people sign in and sign out basically contains logs, so it's called logbook. So it's also valid to use "login and logout" instead of "sign in and sign out". But in those days, "login" and "logout" was not used much mainly because of the fact that you are signing; signing is the most important event there.
When computers came in 1980s, the security is the most important thing, so most of the operation that user do are logged in the system. There are error logs, security logs, transaction logs and many more.
So when a user is allowed into a secured website by taking the username and password, this event is also logged in the system for security reasons. So technically speaking, it's meaningful, valid and better to call this events as "login and logout" instead of "signin and out" because you don't actually sign. But many still use "sign in and sign out" as well just following the old days. But you can see here, the use of "login" peaked after 1990. Logoff is just a synonym of logout.
Whereas Signoff is a phrasal verb having following meanings:
To announce the end of a communication; conclude.
To stop transmission after identifying the broadcasting station.
Informal To express approval formally or conclusively: got the Congress to sign off on the tax proposal.
Sign out means:
To record the departure of another or oneself by signing a register.
So "logoff" and "logout" mean the exact thing when we log out of a computer system or web. "Sign off" or "sign out" is also same in terms of the specific meaning of end of communication by signing. When we logout, a log is registered with several information in the websites to keep track of a user. So all of them are correct in terms of usage in websites.