It can be useful to think about this in terms of the following varieties of sets:
A: A set of values allowed by a computer type (example constraint: int ranges from -2^31 to 2^31-1)
B: A set of values allowed by a user interface (example constraint: the multiplicity of an item in a shopping cart should be non-negative)
C: A set of values allowed by a real-world domain (example constraint: a delivery order has a weight limit)
B - C (set difference) are "unwanted" values, while A - C (and A - B) are "illegal" values. MISU is based on A being under your control as a programmer, allowing you to freely try to reduce it. But B and C are constrained by your requirements, so reducing B isn't simply a computer programming problem, but instead a user experience problem, not addressed by MISU.
It can be useful to think about this in terms of the following varieties of sets:
intranges from -2^31 to 2^31-1)B - C (set difference) are "unwanted" values, while A - C (and A - B) are "illegal" values. MISU is based on A being under your control as a programmer, allowing you to freely try to reduce it. But B and C are constrained by your requirements, so reducing B isn't simply a computer programming problem, but instead a user experience problem, not addressed by MISU.