hfss solve inside
solve inside - implies exactly what it means. for instance if your conductor is of the planar style, you would not be able to solve inside. also attached to that same idea is if your conductor is of finite thickness but you defined a perfect E boundary (whether boundary defined or PEC defined) there would be no need to solve 'inside'.
however if your conductor is of finite thickness and not PEC or perfect E (copper for instance), you may wish to solve inside to consider skin effects and so forth.
normally this is not needed, however in some cases that may be. i'm not sure of your particular design, ie frequency so forth where skin effect, losses etc may have to be accounted for.
i would pay attention to your meshing if you have differences between solving inside versus not. you may have a meshing issue if experiencing radically different solutions (IMO) and you do not expect them.