Word Meaning – Differences Between “Prevent” and “Avoid”

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.

"Mandatory" means that the thing must be done due to some reason or rule.

These courses are mandatory.

That is, you must take the courses or else you fail.

Wearing helmets was made mandatory a few years ago.

It is a rule that you must wear helmets.

"Indispensable" is that the thing is so critical, useful, or important that you cannot conceivably throw the thing away.

The volunteers' help was indispensible.

The mission would have failed if the volunteers didn't help.

He made himself indispensible to the parish priest.

He was deemed a must-have person because of some good quality.

These great tools are so versatile that they are indispensable.

The tools are so good that you shouldn't be without them.

To sum up, these two words are not interchangeable - they have their own distinct meanings.