Difference between instadmg and instaup2date?

What's the purpose of each? Going through the quickstart guide, the instadmg bash script isn't even used, so I'm a bit confused on this note. Are packages that I put in the CustomPKG folder installed when I run InstaUp2Date? Or do I need to run the instadmg bash script to get those to install? I'd like to try most of this stuff before asking here, but I'm confused enough where I don't think I can do that. (e.g. tried restoring a "vanilla" image created by InstaUp2Date, which resulted in a non-booting machine). Any help is appreciated. :)