Angling a model to a directiona light

I’m just trying to get a handle on everything at the moment and one of the first things I’ve tried to do is add a directional light. With little experience, it’s hard to just punch in the numbers needed for the angle you want so I tried to add an arrow model to indicate the position so I could better adjust the values and (hopefully) gain a little understanding. Upon trying to do this, it’s obvious that spatials use quaternions, where the light uses a unit vector, leading me to ask:

Why does the directional light use a unit vector for it’s direction when everything else uses Quaternions? Does it have something to do with the light not having a position? (Direction Vs Rotation?)
And: How should I actually set a model’s rotation to match a directional light?

After going through the math for dummies slideshow a few times, I can’t find anything to help me with this, but I feel like the information is there somewhere.

I’d appreciate any light anyone can shed, I’ve really reeled myself in from creating crazy things to try and get a better understanding of the engine and ran into this tiny stumbling block immediately so my hairline will thank you for any help you can give.

A vector3f is sufficient to denote direction. That’s all that’s needed for a light. (You don’t care which way is up or sideways.)

A quaternion or some similar structure is necessary to denote orientation. In that case, you do also care about which direction is up or sideways.

There is an Arrow mesh already that takes a vector and would save you the trouble of worrying about the quaternions.