$iif is an inline condition.
Code:
var %a
if ($1 == 1) %a = cow
else %a = dog
echo -a %a is an animal

;is equivalent to

echo -a $iif($1 == 1,cow,dog) is an animal
$iif(condition,true,false) - if the condition is true, it returns the 'true' parameter, otherwise returns the 'false' one

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