Control which sub-projects are built in a batch build

In an hzp project file, can I select which sub-projects are built in a batch build?

The help for crossbuild says:

You can do a batch build using the -batch option:

crossbuild -config "ARM Debug" -batch libc.hzp 

This will build the projects in libc.hzp that are marked for batch build in the configuration ARM Debug.

But, I can't figure out how to "mark projects for batch build".