What is the difference between atomic mass unit (amu) and relative atomic masses

I know that the atomic mass is the average mass with isotopes. Then suddenly my textbook talks about the amu. Aren't they the same thing? If so, why are they called differently?? This textbook is confusing me!

The atomic mass is sometimes incorrectly used as a synonym of relative atomic mass, average atomic mass and atomic weight.
These differ subtly from the atomic mass.
The atomic mass is defined as the mass of an atom, which can only be one isotope at a time and is not an abundance-weighted average as in the case of atomic weight.
An atomic weight (relative atomic mass) of an element from a specified source is the ratio of the average mass per atom of the element to 1/12 of the mass of an atom of 12C.
and The atomic mass is the mass of a specific isotope, most often expressed in unified atomic mass units.
The atomic mass is the total mass of protons, neutrons and electrons in a single atom.

I hope it can help you.. Good Luck :)

The amu is a unit of measurement, equal to one gram divided by Avogadro's number.
Equivalently, it is 1/12 of the mass of a C-12 atom (i.e. 6 protons + 6 neutrons + 6 electrons).

(Average) atomic mass is, as you've said, the average mass of an element, taking into consideration the ratio of isotopes.

So they are not the same thing.