While exosomes are best defined as ..., ... was also described.

С экзосомами путаница в определениях [1], так что "классически", НМВ, означает, что дальше идет наиболее общепринятое/наилучшее определение.

[1] Since the original description of exosomes over 30 years ago, the term has been loosely used for various forms of extracellular vesicle, muddying the field and contributing to the scepticism with which the research has sometimes been met.

Exosomes are best defined as extracellular vesicles that are released from cells upon fusion of an intermediate endocytic compartment, the multivesicular body (MVB), with the plasma membrane.
-- https://bmcbiol.biomedcentral.com/articles/10.1186/s12915-01...