[emwa]

Tanzania is actually part of Eastern Africa and not Southern Africa. But you are right, Southern Africa constitutes a group of a countries while South Africa is a single country.

[rmc]

Tanzania can be in both Southern and Eastern Africa. It's in the SADC ( http://en.wikipedia.org/wiki/Southern_African_Development_Co... ).

These terms are just descriptions, there are no hard, objective, definitive rules about it.