The amu without the "unified" prefix is technically an obsolete unit based on oxygen, which was replaced in 1961. However, some sources still use the amu but now define it in the same way as u. In this sense, most uses of atomic mass units or amu today actually refer to unified atomic mass units or u. For standardization a specific atomic nucleus had to be chosen because average mass of a nucleon depends on the count of the nucleons in the atomic nucleus due to mass defect. This is also why the mass of a proton by itself is more than 1 u.
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