# Inertial and Non-inertial frames of reference

I'm really beginner in physics and I recently started to study the concept of frames of reference, inertial and non-inertial ones.

In the end, I thought I had understood it: frames of reference have coordinate acceleration (dependent on other frames of reference) and proper acceleration (universal) .
Proper acceleration is measured by accelerometers, and we can detect how non-inertial a frame of reference is by detecting its proper acceleration via accelerometers.

But then someone talked to me about Principle of Equivalence and not possibly being able to identify what is proper acceleration and what is coordinate acceleration with an accelerometer. Is it true ?

Is there any constraint situation under which we can identify between those, or similarly, can we talk about degrees of accuracy in distinguishing between those two ( a degree of accuracy or a constraint situation in which we can detect the other degree of accuracy in which a frame is non-inertial ? )

If there is such constraint situation or degree of accuracy, then I'd be safe to know that all the talking about frames of reference, non-inertial and inertial would have a use ( inside that constraint situation/degree of accuracy ) .

But if there is no such constraint situation/ degree of accuracy measurement , that is, if we can NEVER distinguish between the proper acceleration and coordinate acceleration and there is not a concept of degree of accuracy of the distinguishment, it seems to me that all this talk about frames of reference, inertial and non-inertial ones, is a bit useless.

As I said, I'm really beginner in physics and I'm not aware of more advanced physics ( general relativity, principle of equivalence, etc ) , i just wanted to know if in fact i should drop all this notion of frames of reference, inertial and non-inertial ones, because in fact this is all universally flawed, or if there are degrees of accuracy or constraint situation in which it is valid/reasonable.