The reason it has a separate texture slot is because you can use multiple UV layouts per mesh.
So for example you could have the diffuse or normal UV charts/islands scaled way beyond the 0-1 range to get tiling or even have several charts/islands on top of each other sharing texel space , but for the AO you then create a second UV layout within the 0-1 range with no overlaps
Look at this model of mine :
HHV ATE® Ballistic Helmet Multicam
Take the straps as an example , here are the two UV layouts used. The diffuse does not even use a texture, its a flat color , the surface detailing comes from the cavity and normal maps. They use the UV set on the left so they tile and look hi-res when zoomed in and it does not take long to load. The AO is on the right and as I mentioned, all islands are separated and fit within the UV boundaries.
Your model may have a mixture of materials with single UV sets and many many with multiple UV sets. When you upload and assign a texture into a slot of a material that has multiple UV sets , sketchfab will display a selection list where you can choose which set to use. I think they support up to 4 unique UV sets per material
So now you know why and how to use it