Currently I'm reading Magnetostatics. While reading Divergence of B, I fell into a confusion that what divergence really means w.r.t. a coordinate system. Supposed there is a current distribution J at r' w.r.t. some primed coordinate system. And B is defined at position r w.r.t. unprimed coordinate system. Now if we integrate (Del.B ) over a volume enclosing the whole current distribution w.r.t. the unprimed coordinate system, then it means the total outward flux over the volume. Upto this line I have no problem. My confusion arises at the following. WHAT (Del.B ) MEANS (no integration over any volume)?? Here "Del" and B is defined from unprimed coordinate system and inside B, J is from primed coordinate and R (in B ) is equal to (r - r'). If it means flux over an infinitesimal unit volume then what is the position of that volume. Thanks.