Here's the definition:

“Verification threshold” means on a rolling consecutive 365 day basis, 15 or more work days missed due to sickness.
Note one: For a pilot who transitions to temporary or long term disability, work days missed due to sickness during the rolling 365 day period preceding his transition to temporary or long term disability will not be counted towards the verification threshold.
Note two: Work days missed during a sick occurrence in excess of 20 consecutive calendar days will not be counted towards the verification threshold provided the sick occurrence was timely verified under Section 14 F. 2. b. and so long as the sickness included:
a. surgery,
b. hospitalization,
c. fractured bone that prevented a pilot from exercising the privileges of his FAA medical certificate, or
d. a serious medical condition which, in the sole discretion of the Director – Health Services, is confirmed through objective pathology, and the pilot provides a medical release under Section 14 G. even if the pilot is not otherwise subject to a medical release threshold.
Note three: Work days missed upon the exhaustion of FAA leave under Section 13 K. will not be counted towards the verification threshold.