Units are items that you can equip or slot into armor, it has some vague similarity to gemming in World of Warcraft. However, you don’t have multiple armor pieces. Instead you just have “Armor” and that armor can have a differing number of slots to equip units into. Slotting also doesnt consume units and you can slot, unslot and swap units about all you like.
In PSO and Phantasy Star Portable 2, there were no unit-type restrictions (except for on special ability units in PSP2). In PSU, unit types were introduced so units could only be slotted into armor which had matching slots. In all these, units were slotted into armor. Well now, in PSO2, things seem to be quite different.
It seems now that you actually slot units into your outfit rather than your armor. There are different unit slots (arms, back, legs) but it’s not known if there will be restrictions on what unit you can slot where. Nor if some outfits might actually have no slots in certain places, which if there were also restrictions would be incredibly bad news for anyone who’s particular about their character’s appearance. For example; you are a techer, you need an arm unit that has a TP boost that only goes in an arm slot but your outfit (which is a damn snazzy lookin outfit for your character) doesnt have one.
Another thing that’s new is that high-level units will now affect your appearance, too. Wether the actual visual effects will be per-costume or regardless of costume is not known, but again it could be bad news for people who care a lot about appearance. Say you’re that Force again, you need a TP unit but it has a graphic that utterly clashes with your outfit/color scheme. Again you’re sacrificing appearance for stats. If the appearance is unique to a given costume of course this won’t be an issue. The screenshots seem to suggest that this is the case, in particular as the units appear to all match their respective character’s color scheme, but then if you don’t want your character with bulkier parts attached to them then it seems like you’re out of luck.
So it would seem that gear changes your appearance, which is a first for a Phantasy Star online game (well if you don’t count weapons and mags in the field). The good side of this is now you can show off your gear in public, which is something you couldn’t really do before. In particular to show off the more difficult to aquire equipment. However, the down side is you may end up having to look like a clown (albeit a sharp pointy metal clown) just to have decent stats. Plus you have the issue that many mmos have where all gear determines appearance, where you have “best-in-slot” or optimal items which everyone will gravitate towards, meaning every character ultimately ends up wearing the same outfits. I somehow doubt that PSO2 will have this problem, but it could have appearance clashing issues if they’re not careful with how they handle this.