Documentation:OCLOperators value

Returns the the non-null value of a simple nullable type.

Expression	Result
int? with the value null	0

The value of the current nullable object is returned if it has been assigned a valid underlying value otherwise 0 is returned.

Some expressions may require a non null value and yet your attribute is a nullable type. A scenario like this is catered for in MDriven OCL expressions.

For example

To calculate the Total Price of all products

Product.allinstances.Price->sum.Value

or

Product.allinstances->collect(i|i.Price.Value)->sum

This will ensure a value is always returned by the expression either when there are no product instances or product price is not set for all instances