What is the reliability of "5 9" (99.999%)?

The high reliability of the software system (also known as usability, described in English as Ha,high Available) has a standard for measuring its reliability--x 9, the X is a representation of numbers. X 9 means that in the software system 1 years of use, the system can be normal use of time and total time (1 years) ratio, we have the following calculation to feel the next X 9 at different levels of reliability differences.
3 9: (1-99.9%) *365*24=8.76 hours, indicating that the software system in continuous operation for 1 years time the maximum possible business interruption time is 8.76 hours.
4 9: (1-99.99%) *365*24=0.876 hours =52.6 minutes, indicating that the software system in continuous operation for 1 years time the maximum possible business interruption time is 52.6 minutes.
5 9: (1-99.999%) *365*24*60=5.26 minutes, indicating that the software system in continuous operation for 1 years time the maximum possible business interruption time is 5.26 minutes.
Then x 9 x only represents the number of five, why no two, nor more than 6 of it. We then calculate:
One 9: (1-90%) *365=36.5 days
2 x 9: (1-99%) *365=3.65 days
6 x 9: (1-99.9999%) *365*24*60*60=31 sec
You can see that 1 9 and 2 9 represent the time of day when the business may be interrupted for 36.5 days, 3.65 days, and the reliability of this level may not be enough to use the word "reliability", while 6 9 means that the maximum business interruption time is 31 seconds within a year, then the reliability of this level is not realized. But to achieve from the 5 9 "6 9 of the reliability of the increase, the latter need to pay more than the former several times the cost, so in the enterprise everyone only talk about (a) 9.