40 Pa. Stat. § 785
The board of directors of such associations may divide the subscribers into groups, in accordance with the nature of their business and the probable risks of injury therein. In such case they shall fix all premiums for each business in such group, and for the various classes of employment therein, in accordance with the probable risks of injury to the employes in such business and in each class of employment therein; and they shall make all assessments, and determine and pay all dividends, by and for each group in accordance with the experience thereof; but all funds of the association and the contingent liability of all the subscribers shall be available for the payment of any claim against the association: Provided, however, That (as between the association and its subscribers), until the whole of the contingent liability of the members of any group shall be exhausted, the general funds of the association and the contingent liability of the members of other groups shall not be available for the payment of losses and expenses incurred by such group in excess of the earned premiums paid by the members thereof.
40 P.S. § 785