From a recent MEC message line:

Operational Extension and the Administration of a Fatigue Call:

Pursuant to Section 12.C.5.b., given extenuating circumstances a pilot may be extended beyond operational limitations, but if a pilot becomes excessively tired he is to call (1) CRS and (2) the Duty Officer. The Duty Officer has two alternatives, and these are listed under 12.C.5.b.ii.(a) & (b).

These alternatives are: The pilot can be scheduled for an alternative duty that falls within operational limits or, be released into a legal rest period. If the pilot requires additional rest, in addition to the rest period already provided, the pilot will be removed from the remainder of his trip and be placed into sick leave.

What is important is that a sick bank deduction for fatigue, when scheduled beyond operational limits, can only be made if it is deemed that additional rest is required.