The operative word is 'protection' and refers to what the system can
guanrantee it can bound the error to with a very high degree of
certainty. The integrity requirement says that there must be no HMI
99.several 9's% of the time. The actual error will vary (due to
satellite geometry, ionospheric activity, satellite(s) going out of
tolerance or being taken out of service for maintenance). WAAS must
detect and either correct or flag.

For more details than most would probably ever want to know, check out
the Tech Center PAN reports.
http://www.nstb.tc.faa.gov/reports/pan47_1004.pdf for example. Amongst
the plethora of statistics and charts, there's some good explanations
of the phrases being tossed about, such as "protection level" and
"HMI".

Regards,
Jon

Regards,
Jon