... and Project Management

Often confused, portfolios are different than programs. Portfolios are a management instrument and they may therefore contain projects that are not directly related. For example, limited on-site management capabilities of a geographical location may require to assign the responsibility for all kinds of projects to one person or group.   

... and Complexity

Obviously, if enterprise projects are complex, portfolios are as well. They reflect another level of complexity of "project systems," in the context of "self-similar" structures of project management.