What does an availability of "five$-$nines" $(99\mathrm{.}999\%)$ signify in disaster recovery?

The system can recover from any disaster in $99\mathrm{.}999\%$ of cases.

The system requires maintenance $99\mathrm{.}999\%$ of the time.

The system is unavailable for $99\mathrm{.}999\%$ of the time.

The system is available for $99\mathrm{.}999\%$ of the time.