The PS2 Mana Khemia games are also considered to be 'Atelier spin-off' games...
So confusing!
It really seems more daunting than it is
To distill the Ar tonelico series & the Atelier series of games into what it is that differentiates them the most from each other; go with:
Ar tonelico games = more like traditional, turn-based RPG's with a huge emphasis on Lore, fleshing-out characters w/ awesome backstories, and some of the best music...ever (seriously - they made up an entire language for the Ar tonelico world (Hymnos) that many of the songs are actually sung in...IMO, the quality *in spite of being unable to understand the lyrics* (no - I'm not one of those "fans" who's actually *learned* that language; (you'd be surprised at how serious some fans are:)) stands out even more, I think.
Atelier games = Much more light-hearted stories (think a sister finding her lost sister/looking for a mother that disappeared) rather than a "we need to band together for the sake of all that is decent to save the world") kind of themes. Combat in Atelier games is almost an afterthought & more of a means to an end (that end being to go places & gather ingredients & materials). The "Heart" (read: what you'll be doing for ~75% of most Atelier games) is *
Synthesis*.
It's all about the items that you make!
To the extent that only the main characters (Rorona, Totori, Meruru, Ayesha, etc) can even "use item" as a battle option.
Actual stats are kind of put on the back burner since you make all of your own equipment, recovery items, buffs/debuff items, revive items, bombs/damaging items, etc yourself...the synthesis system is actually pretty complex & in-depth & has a bit of a learning curve -but- just think that, instead of stats raising every time you level up...you'll accomplish the equivalent by learning & becoming able to create better items - which, in turn - basically accomplishes the same thing.