The distinction between AUVs and other types of marine vehicles is not straightforward. The field of taxonomy in biology faces a similar issue - what defining characteristics should be used to group different species? Biologists use a number of different methods - differences in size, rRNA, cell structure, life history, ecological (functional) role, etc. - and each biologist has his/her preferred classification method(s). Similarly, marine robotic vehicles can be classified by differences in size, propulsion, energy acquisition method and storage, depth capacity, decision-making abilities, etc.

Taxonomical definitions are also evolving; for example, many are familiar with the older "Kingdoms" system (which included the two, five, and six kingdoms) and (are possibly less familiar with) the "Three Domains" classification, which suggests that life can be classified by an organism's cell structure. An even earlier classification divided life into two types: prokaryotes, which are unicellular and do not have membranes to contain their organelles, and eukaryotes, which may be single or multicellular and have membranes. In 1977, an American biologist named Carl Woese suggested that prokaryotes be split into two groups: bacteria (containing all "typical bacteria") and archaebacteria (which, in 1977, only comprised of methanogens, or bacteria that could produce methane, but now includes more members and has been renamed to archaea), resulting in the "Three Domains" [1]. However, to this day, biologists continue to debate the various classification methods, with continued modifications occurring as we learn more about life on Earth. Likewise, marine robotic enthusiasts will each have their own methods of classifying marine vehicles.