Study options

I everybody,
I've been using COMSOL for more than a year now, and up until now I had no need to changing anything in the default solver, but now I do.

To be more specific, I work in a huge file which has three models inside it, and accordingly I use three different Study nodes, one for each. Two of the three models also combine more than one physics. For some reason, the Dependent Variables in each of the solvers also contain the dependent variables of all the models, so I have to open it up and mark out whichever variable I do not want it to solve for or present in output.

I have two questions then:
1. Why does it show all the variablles in each of the studies? can one study compute for all models together, each with its own Physics, geometry and mesh? What is the meaning of all that?

2. Does it matter from where I compute? When I change those variables I always use 'compute all' from the Time Dependent Solver node that is beneath the Dependent Variables node. If, after I edit the DV node, I hit the Study note and Compute All - will it override anything I changed, or will it do the same?

Thank you very much once again,

Shoval~