It is a cylinder provided with one or two bunches of sealed keys (1st, 2nd and 3rd use).
The 1st bunch is normally used to open the cylinder.
Should any of the keys in the 1st bunch get lost, security is guaranteed using a key from the 2nd bunch. The insertion of the second-use key disables the first –use key (the process is not reversible).
If a cylinder is provided with the 3rd bunch, the lock can be further recoded.