Another wording of FAR 25
3. In high altitude mode and for operation over 25 000 ft, the warning altitude setting is such that corrective actions can be taken in time to insure that cabin pressure altitude can not exceed 15,000 ft "in the event of any reasonably probable failure" of the Pressurization System.
http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgELOS.nsf/0/eb3fc2b4897f9c05862574a3005cdf07/$FILE/TC0030IB-T-%20SE-13.pdf