reftex-use-multiple-selection-buffers

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

nil

Documentation:
Non-nil means use a separate selection buffer for each label type.
These buffers are kept from one selection to the next and need not to be
created for each use - so the menu generally comes up faster. The
selection buffers will be erased (and therefore updated) automatically
when new labels in its category are added. See the variable
`reftex-auto-update-selection-buffers'.

You can customize this variable.