"This is not always used." vs. "This is not used very often."

Question

Are the above sentences interchangeable?

Do they have the same meaning or there is a little difference between them?

What is the most common way?

Answer 1

Are the above sentences interchangeable?

Definitely not.

I would use "This is not always used." when talking about something that is almost always used, but not always.

I would use "This is not used very often." when talking about something that is almost never used, but is used sometimes.