Khawar Ahmed has written a good article describing installation scenarios for the Atlantic wave of Eclipse-based IBM software engineering products. It explains how the different products relate to each other in terms of sets of functionality, and also describes how different products can co-exist, or not, in some cases. See the graphic below (taken from the article) for a nice visual representation of the different sets of functionality in the products.
Eclipse-based IBM Rational product packaging (from Khawar Ahmed article)
I understood some of this stuff intuitively from experience with the different products, but Khawar's article provides a nice explicit reference which I'm sure I'll find myself sharing with others as they explore the different tools available to them.
One interesting thing to note is that the Software Architect product includes the Application Developer product as a subset of its functionality. This naming may be a tad controversial since in some development cultures it is expected that architects design AND code, and in other cultures architects are responsible for high-level design but NOT implementation. That Software Architect includes all of the Java/J2EE development functionality, and that there is a separate product (Software Modeler) that provides just the high-level design (UML modeling) functionality, tells you where Rational stands in this nomenclature debate.
Check out this anti-pattern for some arguments why architects not coding is a bad idea.
Good article on Eclipse-based IBM product packaging and installation scenarios