A = 1 < 10;
mustBeNonpositive(A)

Error using mustBeNonpositive (line 14)
Value must not be positive.

Because the expression 1 < 10 returns logical 1, the value of A is positive and causes mustBeNonpositive to issue an error.

This class restricts the value of Prop1 to a nonpositive values.

classdef MyClass
   properties
      Prop1 {mustBeNonpositive}
   end
end

Create an object and assign a value to its property.

obj = MyClass;
obj.Prop1 = 10;

Error setting property 'Prop1' of class 'MyClass':
Value must not be positive.

When you assign a value to the property, MATLAB calls mustBeNonpositive with the value being assigned to the property. mustBeNonpositive issues an error because the value 10 is positive.

function r = mbNonpositive(lower,upper)
    arguments
        lower {mustBeNonpositive}
        upper {mustBePositive}
    end
    x = lower*pi:upper*pi;
    r = sin(x);
end

Calling the function with a value for lower that does not meet the requirements of mustBeNonpositive results an error.

r = mbNonpositive(2,4);

Error using mbNonpositive
Invalid input argument at position 1. Value must not be positive.