This thread makes me feel retarded...
Hahaha ... you're not, we're just throwing around acronyms to make everything
seem more difficult than it is.
It's all just the details of how the System Card goes about loading and running your game from CD when you respond to the
"PUSH RUN BUTTON!" message.
It's all just boring (but critical) details ... and the big thing to note is just that "IPL" is shorthand for "Initial Program Loader". It's called that because it is the first thing that is loaded (and run) from the CD that then loads up your (the developer's) game code.
It's a small program that's put in the very first sector of the CD Data Track, and it is often also refered to as the "signature".
It is really just a piece of Hudson's copyrighted code that
must appear at the start of every CD game, or else the System Card will refuse to run the CD.
The IPL looks at the information that you (as the developer) put into the 2nd sector on the CD to tell it where to load your game program into the PCE's memory, and exactly where it is on the CD.
It's not too difficult a concept to understand, it's just the details that can make it a big ugly.
PCEAS/HuC hide all the nasty stuff from you, and just make it easy for a newbie developer to get something running.
The details are all described in the official "Hu7 CD System" developer document that's been knocking around the interwebs for the last few years, as Punch eventually discovered.