Variable: TeX-default-mode
TeX-default-mode is a customizable variable defined in tex.el.
Value
LaTeX-mode
Documentation
Mode to enter for a new file when it can't be determined otherwise.
Probably introduced at or before Emacs version 18.
Source Code
;; Defined in ~/.emacs.d/elpa/auctex-14.1.2/tex.el
(defcustom TeX-default-mode #'LaTeX-mode
"Mode to enter for a new file when it can't be determined otherwise."
:group 'TeX-misc
:type '(radio (function-item LaTeX-mode)
(function-item plain-TeX-mode)
(function :tag "Other")))