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A single equals sign "=" is used to perform an assignment, so it is the equivelent of the to operator.
(set: $variable = "value")
(set: $variable to "value")
A double equals sign "==" is used to perform a comparison, so it is the equivelent of the is operator.
(if: $variable == "value")
(if: $variable is "value")
a. A triple equals sign "===" can also be used to perform a comparison if the data-types of the two things being compared are the same.
b. Because people often mistakenly use the wrong number of equal signs when doing assignment or comparison I generally recommend using the to and is operators instead.
So your example is actually assigning the values 11, 22, 33, 44, 55, 66, 77, 88, and 99 to the $SkillRoll story variable, not compating that variable to those values.
(if: $SkillRoll <= $ASkill and $SkillRoll % 11 is 0)[something happens]