The AMP settings screen describes the three modes: Reader, Transitional, and Native.

Originally Reader was called “Classic” and Transitional was called “Paired”, but these were renamed in #2034. The “Native” name was not changed, however. While it used to be called “canonical AMP” it is now termed “AMP first” as referenced in AMP as your web framework. While the Native mode description does reference AMP-first, the mode itself is not termed that yet. This should change.

Changing the mode name may need to be done as part of a redesign of the whole screen, as with #1288. This especially true as we hope to be able to default sites to AMP-first in the future (as opposed to Reader mode), per #1384.