For purposes of this part, an “amount recoverable” is an amount of unemployment, sickness, or maternity benefits paid under the Railroad Unemployment Insurance Act which is:
(a) Determined to have been paid erroneously;
(b) Recoverable under section 2(f) of the Act because remuneration is found to be payable with respect to a period which includes days which had been determined to be days of unemployment or sickness;
(c) Recoverable under section 4(a-1)(ii) of the Act because of the employee's having received or been held entitled to receive annuity payments under the Railroad Retirement Act, insurance benefits under title II of the Social Security Act, unemployment, sickness or maternity benefits under any law other than the Railroad Unemployment Insurance Act, or any other social insurance payments under any law; or
(d) Recoverable under section 12(o) of the Act by virtue of the Board's right to reimbursement from any sum or damages payable through suit, compromise, settlement, judgment, or otherwise on account of liability based upon an infirmity, to the extent that it will have paid or will pay benefits for days of sickness resulting from that infirmity.