Why does the AOMap use the second UV channel?

[Mugen87]

This is explained in this thread:

Why do AO and LightMap need a second set of UVs?

To summarize this a bit and explain why it applies to AO/lightmaps specifically — three.js does not (yet) provide a proper API for changing which UV set is used by a particular texture. Various proposals and workarounds have been made, but it’s still essentially a missing feature. UVs are designed by artists in order to maximize the amount of texture detail in specific areas of the mesh. For most textures, bigger areas need more detail. For AO and lightmaps that’s often not the case — you need…