Trigger builds of the other projects once a build is successfully completed. Multiple projects can be specified by using comma, like "abc, def".
Other than the obvious use case where you'd like to build other projects that have a dependency on the current project, this can also be useful to split a long build process in to multiple stages (such as the build portion and the test portion).
Nested projects need to be specified by using a path. It can be an absolute path starting from Jenkins root, like "/grp/abc" where "grp" is located at the root of Jenkins. It can also be a>