Strength is one of the properties of a Cyc assertion. Its possible values are “has exceptions” and “currently exceptionless”. At the heuristic level, i.e., once assertions are stored in the KB, strength is combined with the EL truth value (negated or not) to produce 4 composite truth values: default true (true but with exceptions), monotonically true (true and (currently) exceptionless), default false, and monotonically false. Together with “unknown”, these form the representations of truth at the HL level.
When a CycL sentence is originally asserted into the KB, its strength will be “currently has no exceptions”. Assertions that gain exceptions will have their strength adjusted to “has exceptions.”
At the epistemological level, it is possible to declare whether exceptions are expected for an assertion or not using #$willNeverHaveExceptions and #$hasExceptions. Assertions with these meta-assertions will be displayed in the browser as “necessarily monotonic” and “has unspecified exceptions,” respectively. #$willNeverHaveExceptions is used by the WFF Checker to prevent exceptions from being added to that assertion. #$hasExceptions is essentially only documentation; it is a placeholder for future exceptions a Cyclist expects an assertion to have, but which haven’t yet been stated.