Whenever you have a bulky base, for example t-butoxide, then you do an elimination. You cannot do an SN2 with a bulky base simply because of steric hindrance. You should form the MOST stable (more substituted) alkene.
I'm not sure what you mean by bulky halide. Do you mean the reactant? When the reactant is tertiary, no SN2 can occur because, again, of steric hindrance. Therefore, you resort to E2.
Is this clear?