Not obligate; optional, discretionary or elective.
That grants permission or power to do something.
Able to perform a particular life function, or to live generally, in more than one way.
Of or relating to faculty, especially to mental faculty.
At which a given function is positive.
Not compulsory; left to personal choice; elective.
In various programming languages, a kind of variable that is assigned a specific data type but may or may not hold an actual value.
Something that is not compulsory, especially part of an academic course.