Plotinus, when he thinks about mind or intellect, the Greek word is 'nous', he thinks about something that's very different, it's much more elevated and special, more abstract, you might say more philosophical than the very broad range of mental events that we talk about in contemporary philosophy of mind.

Soul puts the determination or forms or images of forms, into matter.

Who you really are, is an immaterial soul and the body is an external thing that's sort of an encrustation your soul. So this has important implications for Plotinus' ethics, because his ethics are basically all about encouraging us to turn away from the body and turn towards these higher principles, so universal soul, universal intellect and ultimately the One.

Plato in his dialogue The Phaedo says that whereas sticks and stones are both equal and unequal, (so maybe what that means is that each stick is going to be equal to some other sticks and unequal to some other sticks, so equal to the stick on the left maybe but shorter than the stick on its right) the form of equal is going to be just equal, and it won't partake of inequality at all. And it will be the cause of equality in things that are equal, for example, equal sticks and stones.

The forms in intellect are some kind of blueprint or model on which the physical world is based, and something needs to come along and shape the matter in accordance with that blueprint. What does that is soul.

What the soul is doing is kind of walking through the forms, and so our experience of thinking isn't normally this kind of pure intuitive insight that intellect gets, and that intellect must get right, because it's always identical to its objects, it's always the same as the forms that it's thinking about.

Assuming there is an intellect, we're clearly not this universal intellect or we would know it. So that's one function of soul.