Hi Rae,
I am not sure I understand your question correctly.
A dependence cannot be mandatory and discretionary at the same time; similarly, external and internal are mutually exclusive attributes. However, mandatory and internal are not mutually exclusive, so I don't see why to choose between the two.
Also, keep in mind that some dependencies may not fall in one of the two categories per se, but depending on the context. A dependency is not something "absolute", but it expresses a logical relationship between two activities. Do you have a specific example in mind?
Gabriella