reftex-insert-label-flags

reftex-insert-label-flags is a variable defined in `reftex-vars.el'.
Its value is

("s" "sft")

Documentation:
Flags governing label insertion. First flag DERIVE, second flag PROMPT.

If DERIVE is t, RefTeX will try to derive a sensible label from context.
A section label for example will be derived from the section heading.
The conversion of the context to a valid label is governed by the
specifications given in `reftex-derive-label-parameters'.
If RefTeX fails to derive a label, it will prompt the user.
If DERIVE is nil, the label generated will consist of the prefix and a
unique number, like `eq:23'.

If PROMPT is t, the user will be prompted for a label string. The prompt will
already contain the prefix, and (if DERIVE is t) a default label derived from
context. When PROMPT is nil, the default label will be inserted without
query.

So the combination of DERIVE and PROMPT controls label insertion. Here is a
table describing all four possibilities:

DERIVE PROMPT ACTION
-------------------------------------------------------------------------
nil nil Insert simple label, like eq:22 or sec:13. No query.
nil t Prompt for label.
t nil Derive a label from context and insert without query.
t t Derive a label from context and prompt for confirmation.

Each flag may be set to t, nil, or a string of label type letters
indicating the label types for which it should be true. The strings work
like character classes.
Thus, the combination may be set differently for each label type. The
default settings "s" and "sft" mean: Derive section labels from headings
(with confirmation). Prompt for figure and table labels. Use simple labels
without confirmation for everything else.
The available label types are: s (section), f (figure), t (table), i (item),
e (equation), n (footnote), N (endnote), plus any definitions in
`reftex-label-alist'.

You can customize this variable.