reftex-auto-update-selection-buffers

reftex-auto-update-selection-buffers is a variable defined in `reftex-vars.el'.
Its value is

t

Documentation:
Non-nil means, selection buffers will be updated automatically.
When a new label is defined with `reftex-label', all selection buffers
associated with that label category are emptied, in order to force an
update upon next use. When nil, the buffers are left alone and have to be
updated by hand, with the `g' key from the label selection process.
The value of this variable will only have any effect when
`reftex-use-multiple-selection-buffers' is non-nil.

You can customize this variable.