The default value of LaTeX-section-label is:
(("part" . "part:")
("chapter" . "chap:")
("section" . "sec:")
("subsection" . "sec:")
("subsubsection" . "sec:"))
However, for chapter, when typing C-c C-s, we get cha instead of chap:.
\chapter{asdf}
\label{cha:asdf}