Network Uptime Warranty
Some web hosting companies market their plans as 99.9% Network Uptime Warranty. However, in the practice, this doesn't mean your site will be up 99.9% of the time or that you really have any warranty.
What does it mean for you? In case of a server failure or a system crash, according to your hosting company they still have connectivity to the Internet or in other words network uptime.
In a broader sense, a hosting company could claim they refer to their local network connectivity uptime.
In simple words your site can be down several hours due to hardware or system failure and you have no uptime warranty.