A. “Employing unit” means any of the following which has or had in its employ one or more individuals performing services for it within this Commonwealth:
1. Any individual or type of organization, including the state government and its instrumentalities;
2. Any of the political subdivisions of this Commonwealth and their instrumentalities;
3. Any instrumentalities wholly owned (i) by this Commonwealth and one or more political subdivisions, (ii) by a combination of political subdivisions or (iii) by any of the foregoing and one or more other states or their political subdivisions;
4. Any partnership, association, trust, estate, joint-stock company, insurance company or corporation, whether domestic or foreign, or the receiver, trustee in bankruptcy, trustee or successor thereof; or
B. All individuals performing services within this Commonwealth for any employing unit which maintains two or more separate establishments within this Commonwealth shall be deemed to be employed by a single employing unit for all the purposes of this title. Whenever any employing unit contracts with any person for any service which is a part of such employing unit’s usual trade, occupation, profession or business, that employing unit shall be deemed to employ all individuals employed by such person for such service unless such person performs service or is in fact actually available to perform service for anyone who may wish to contract with him and is also found to be engaged in an independently established trade, occupation, profession or business. Each individual employed to perform or to assist in performing the work of any agent or employee of an employing unit shall be deemed to be employed by such employing unit for all the purposes of this title, whether such individual was hired or paid directly by such employing unit or by such agent or employee, provided the employing unit had actual or constructive knowledge of such work.