((If you don’t mind me adding something to this post here, @comyet, but here is a more visual representation of Fusions and Combos!
Using Paper Jam as an example!
* (Second Pic) Original PJ is more like a Combo than a Fusion - considering his story is based around Ink and Error accidentally creating him through all of the leftover ink and strings from their battles. So, he’s more like a kid of the two in the combo spectrum!
* And the third pic is a Fusion Paper Jam! This one is an actual fusion of the two characters! (my personal head-canon with this one is that Ink tried to escape at the same time Error was trying to grab him but he glitched out. That forced the two together and they are now stuck)
When I draw a combo, they always have just two eyes, two arms, two legs - basic body proportions. (though a combo could be lacking some of these things - but it still follows the basic body structure). When I draw fusions, I normally draw them with three or more eyes - typically with four pupils, even if they are in the same eye. They could also have more limbs and they carry more traits of the two characters into the design. Some other fusions I’ve created can be found here and here!
And that is another interpretation on what are fusions and what are combos! Hope that this also helps!))