quantum field theory - How do we know we've unified two interactions?

1. 
   

   What is the precise definition of unification of fields (in classical and quantum mechanics)?

   
   


2. 
   

   In general, does unification of a field mean that we can write both of them at both sides of an equation (like Maxwell's laws)? Or does it mean that one of them can produce the other (like $E$ and $B$)?

   
   



3. 
   

   Is there any intuitive explanation of how electroweak unification works? Like an electric charge will feel a weak field or that a flavoured particle will produce a weak field?