!%= are approximate equality operators.
When the two values placed on the stack are within 10% of the first one (chronologically),
%= places a "true" on the conditions stack.
!%= returns the opposite of
%= and is equivalent to
50 55 %=will place "true" on the conditions stack.
In VisualBasic, the actual operation used to handle
a b %= is (equivalent to)
Abs(a - b) <= a/10, where all operations are performed on Single-precision integers. Therefore,
9 10 %= is false because 9/10 evaluates to 0, while
10 9 %= is true because 10/10 evaluates to 1.