You could almost think of it as being a bonus game in a similar vein to Oh No More Lemmings (except fanmade). The only really special feature as far as the graphic sets go is that they're entirely DOS compatible - Lemmix players bundle the files into the EXE (but it does use the native DOS formats), but I can easily distribute the graphic sets seperately. To the best of my knowledge, this is the first case of custom graphic sets in a DOS-compatible format - this also contributes to them retaining the classic Lemmings look and feel to some extent.
The primary reason for LPII being available only as a Lemmix player, not standalone datafiles which can be used with CustLemm or ONML, is due to special features in other places. There's a few mechanics changes (the most notable one being the instant bombers) and bugfixes (one way right miners, for example), and in turn these levels are built for these mechanics, not those of the original game. A level that required mining through one-way-right terrain or a bomber to detonate within the first 5 seconds, for example, would be impossible under the normal mechanics. Apart from that, there's also the gimmick levels and Floater Frenzies, which the LPII Lemmix player has special code to handle - though these were more a matter of "since I'm using a custom Lemmix player, let's do this" rather than "I want to do this, I'm going to have to use Lemmix".
But yeah. Ultimately, LPII is essentially just a really fancy (and fairly large) level pack contained in its own Lemmix player, with custom graphics, a few mechanics adjustments (mostly fixing bugs, the only one that's a true change is the way the bombers work), and a very small number of levels which deviate slightly from the usual mechanics (as in, the core game is still the same, but minor details change - such as being unable to pause in Floater Frenzy, or the absence of gravity in Who Turned The Gravity Off?, etc). I guess on a continuum, it'd be somewhere between "level pack" and "fan game"... maybe it'd count as a mod? I'm not sure. Have you played the
Lemmings Plus DOS Project? It's basically like that, but with custom graphics and a very few levels with unique features.
And sorry, but LPII is my own project, so I'm not looking for help with the graphic sets or levels - even on the technical side, I'm trying to ask only when I absolutely need to. However, I'm more than happy for you to use my graphic sets to create your own levels and distribute those levels however you like - all of the sets
except desert are included with the second demo (the desert style hadn't even begun being made, let alone finished, when that demo was released). Desert style's files will be released with the full release when it's ready. The graphic sets work as-is with LemEdit or the Lemmix editor; you can use my extractor program (posted over in Fan Corner) to extract them to a format that'll work with jLevelBuilder. If you're using any other editor, you're on your own sorry. =P As for playing them, the graphic sets work fine with all the DOS games*, or the Lemmix editor's playtest mode (or if you know how to compile your own Lemmix player, they'll work fine in that too); or again, you can use that tool to convert them to Lemmini format (the sound effects of the traps will likely be wrong though, because the convertor program uses hardcoded values for their sounds).
* CGA/Tandy versions excluded. However, it should work fine under EGA. (However, in EGA mode, the colors may look weird, as just like LemSet downscales the 24-bit pallete to 18-bit colors for VGA (which is barely noticable), it again downscales them to 6-bit colors for EGA (which most certianly is noticable); they are not currently possible to define seperately and at any rate I see little reason to do so - hardly anyone uses EGA now, and in the unlikely event they have to, the sets are still purely functional, they just might have slightly weird colors.)