LTSP - Automatic build package selection

After recently instaling and setting up an LTSP environment, to learn more about it, using a few older spare PC's I have, I noticed that there are packages missing on the clients. In specific [festival](http://www.cstr.ed.ac.uk/projects/festival) and [sox](http://sox.sourceforge.net/) . At first thought, without knowing much about the ltsp-build-client mechanism, thought it was maybe the fact that the server is 64bit (So possibly had a 64 bit version of these packages installed), that was the reason for LTSP excluding it from the 32bit image. However, after building a 64bit image, and booting it, there was also no trace of these packages files. I ended up having success with chroot'ng into the directory that contained the image files, used apt-get to install a copy that worked on the client, and then ltsp-update-image. However, would really like to know more about the LTSP build mechansim - Anyone know how LTSP decides, out of server installed packages, what to include in a client image, and what not? (I don't suppose order of installation of the three packages in question, matters?)