If the user updates a library, the old version of the library is often being kept because other packages are still being built against this library. emerge could solve this by updating all these packages that are still built against the old version. However, this can have undesired effect like a long installation time when updating a library. So emerge does not update the depending libraries, but put them into a set instead: @preserved-rebuild.
After the update, the user can run the following command if he wishes to clean up the old library version:
emerge --ask @preserved-rebuild