What software? And did they specify a particular material/shader to use? It's usually just a multiplier for the diffuse, like you say, so you need a shader that allows this. If using Max's Standard material, you can put a RGB Multiply map in the Diffuse slot, with your diffuse and ao in its two slots. If using an FX…
So usually when I make an ambient occlusion map I just apply it to my diffuse map and multiply it. I'm doing an art test right now and they're asking me to place it separately. But where does it go?
This is being done in Max. Here's what they said about it: "The diffuse texture for a raceday object should be contained on a single 1024x1024 sheet. Create an Ambient Occlusion map sized 512x512 for the LOD00 model and apply it to all three LODs of the model using a shell material" Is a shell material a shader?
If they're asking you to place it separately then they're probably used to having a shader that utilizes that (although that's a bit redundant since you can just multiply it over all of your maps). If they're having you make the shader in an engine like UE3 just use a multiply node and multiply your diffuse and specular…
It doesn't make much sense. Shell material is built so you can see a different material in the viewport than you see in the renderer. But maybe they've built their export path around it. If you don't want to ask them for clarification (I would if I were you), you could just assume they want you to stick the AO in the…