The presence of a clausilium is the reason for the common name "door snails", because all the snails in this family have a roughly spoon-shaped "door" or clausilium, which can slide down to close the aperture of the shell.
However, this structure is emphatically not the same thing as an operculum, which is virtually non-existent in pulmonate snails, only occurring in the Amphiboloidea.
The wide flat end of the clausilium can close the aperture of the snail shell, and thus protect the soft parts against predation by animals such as carnivorous beetle larvae.
The narrow end of the clausilium slides in a groove, which is formed by spiral folds on the inside of the shell around the columella.
The mechanism is totally different, but the clausilium is vaguely reminiscent of an automated garage door opener.