I see Keith answered about the mechanics while I was typing this.
I’m going to attempt to answer the “How is this supposed to work in xtUML?” question, but probably not very directly. ;-) (One caveat, I’ve never been very comfortable with components and interfaces; from an analysis perspective, they require more effort without adding benefit.)
xtUML as described in the Mellor and Balcer book doesn’t use components and interfaces. In BridgePoint, components are used to represent domain and interfaces represent bridges to/from the domains. UML doesn’t have domains or bridges as defined by xtUML, so something had to be chosen. Originally it was Packages, but Components seem to be a better fit.
Domains are supposed a black box containing a single subject matter in a system. They should already be at their most decomposed level (from an analysis perspective), so if a component is a domain, then subcomponents don’t make sense. The Mellor and Balcer book does talk about subsystems, but they were only added as a way to partition a large class model for a single domain. Subsystems are a graphical tool concept, and aren’t really part of the xtUML modeling “language”.