About extension "Konami Ultimate Collection" in openMSX.

Did you actually select the mapper type in catapult? Looks like it was set on auto detect, which is the default.

Ah, the point of the alternative option was to not use the extension, but only the ROM image in slot a, and specifying the konami ultimate collection mapper type for it. You do know how to set mapper type in catapult? Click the Cart A button.

Can you be a bit clearer on what caused the confusion?

Yeah, no problem. I'm just trying to understand what to explain better.

It's a bit tricky anyway. Extensions are basically hardware cartridges you can put in the cartridge slots. ROM images are special: for them a special flexible kind of extension is created that contains the ROM. But a bare ROM image doesn't say what kind of memory mapper hardware is supposed to be used with it. So you need to specify the mapper type with the ROM image, so the proper extension can be created. This mapper can be specified in 3 ways: 1) automatically, by looking up the ROM in a database that contains the mapper types 2) auto-detect, for games that do not appear in the database (KUC is typical, as you can create your own ROM and 3) manual specification/override. What you now did is 3.

So, the point is that ROM images are treated as special generic extensions. And this is not very much reflected in the Catapult UI (or anywhere else), because it's more convenient to handle them as media instead of extensions...

So what happened in your case in terms of real hardware:
1) you put a KUC cartridge in a cartridge slot, but it wasn't flashed. You can flash it properly of course.
2) you put a ROM cartridge in another cartridge slot which was autodetected as a Konami MegaROM, and the ROM contained the KUC.ROM. But as the mapper was wrong, it wouldn't work properly.
Once you flashed the KUC cartridge, it would simply boot: Catapult inserts extensions first and then the stuff from the "Cartridge slots" tab. So the KUC cartridge ended up in a lower slot than the auto-detected ROM. That explains why it booted once it is flashed, as the MSX will run cartridges from the lower slot first.

I hope that clarifies things a bit.

It's actually in the User's Manual ;-) See the last part of this section: http://openmsx.org/catapult-manual/user.html#software