reftex-section-prefixes

reftex-section-prefixes is a variable defined in `reftex-vars.el'.
Its value is

((0 . "part:") (1 . "cha:") (t . "sec:"))

Documentation:
Prefixes for section labels.
When the label prefix given in an entry in `reftex-label-alist' contains `%S',
this list is used to determine the correct prefix string depending on the
current section level.
The list is an alist, with each entry of the form (KEY . PREFIX)
Possible keys are sectioning macro names like `chapter', section levels
(as given in `reftex-section-levels'), and t for the default.

You can customize this variable.