Which sprite atlas goes into an asset bundle?

I have been searching for an answer to this question but have been unable to find one.

Let's say I pack a bunch of sprites into an atlas A.

I then add half of these sprites into an assetbundle X and the other half into another assetbundle Y.

When I build X and Y:

1. Does A get added to each of them?

2. Or does Unity generate a new atlas for each bundle containing only the relevant sprites?

3. Or is no sprite atlas is included in either bundle?

4. None of the above.

Thanks.