nsubstitute method returns

This value will be returned every time this call is made. Returns() will only apply to this combination of arguments, so other calls to this method will return a default value instead.

//Make a call return 3:
calculator.Add(1, 2).Returns(3);
Assert.AreEqual(calculator.Add(1, 2), 3);
Assert.AreEqual(calculator.Add(1, 2), 3);

//Call with different arguments does not return 3
Assert.AreNotEqual(calculator.Add(3, 6), 3);

Return for any args
A call can be configured to return a value regardless of the arguments passed using the ReturnsForAnyArgs() extension method.

calculator.Add(1, 2).ReturnsForAnyArgs(100); 
Assert.AreEqual(100, calculator.Add(1, 2));
Assert.AreEqual(100, calculator.Add(-7, 15));
Tip! You can also use the default C# keyword for better readability:

calculator.Add(default, default).ReturnsForAnyArgs(100);
The same behaviour can also be achieved using argument matchers: it is simply a shortcut for replacing each argument with Arg.Any<T>().

ReturnsForAnyArgs() has the same overloads as Returns(), so you can also specify multiple return values or calculated return values using this approach.