reftex-format-ref-function

reftex-format-ref-function is a variable defined in `reftex-vars.el'.
Its value is

nil

• This variable may be risky if used as a file-local variable.

Documentation:
Function which produces the string to insert as a reference.
Normally should be nil, because the format to insert a reference
can already be specified in `reftex-label-alist'.

This hook also is used by the special commands to insert
e.g. `\vref' and `\fref' references, so even if you set this,
your setting will be ignored by the special commands.

The function will be called with three arguments, the LABEL, the
DEFAULT FORMAT, which normally is `~\ref{%s}' and the REFERENCE
STYLE. The function should return the string to insert into the
buffer.

You can customize this variable.