reftex-derive-label-parameters

reftex-derive-label-parameters is a variable defined in `reftex-vars.el'.
Its value is

(3 20 t 1 "-"
   ("the" "on" "in" "off" "a" "for" "by" "of" "and" "is" "to")
   t)

Documentation:
Parameters for converting a string into a label.
This variable is a list of the following items.

NWORDS Number of words to use.
MAXCHAR Maximum number of characters in a label string.
INVALID nil: Throw away any words containing characters invalid in labels.
t: Throw away only the invalid characters, not the whole word.
ABBREV nil: Never abbreviate words.
t: Always abbreviate words (see `reftex-abbrev-parameters').
not t and not nil: Abbreviate words if necessary to shorten
label string below MAXCHAR.
SEPARATOR String separating different words in the label.
IGNOREWORDS List of words which should not be part of labels.
DOWNCASE t: Downcase words before using them.

You can customize this variable.