what are elastance(daraf) and electrostatic potential coefficient?
Elastance is the inverse of capacitance. It's not widely used in practical calculations, but it can make theoretical equations more streamlined. It's the same with resistance and conductance. The resistance of parallel resistors is more complex formula than the sum of two conductances. 

Potential cofficient is the elastance expanded to a system of multiple separate charged conductors. With vector integral calculus one can prove that in a group of several separate charged conductors (nothing is moving, the space is otherwise empty) a conductor X has potential, which is weighted sum of the charges of all conductors. The weighting factors are potential coefficients between X and other conductors. One of the charges is Qx = what X itself carries. For Qx the potential coefficient is X's self-elastance against infinity.

All potential coefficients are complicated to solve, but they depend only on the geometry. See this for further reading:

http://nptel.ac.in/courses/115101005/downloads/lectures-doc/Lecture-12.pdf