What is the explanation of Section 2 of the Negotiable Instruments Law?

sec. 2 What constitutes certainty as to sum. - The sum payable is a sum certain within the meaning of this Act, although it is to be paid:

(a) with interest; or

(b) by stated installments; or

(c) by stated installments, with a provision that, upon default in the payment of any installment or of interest, the whole shall become due; or

(d) with exchange, whether at a fixed rate or at current rate; or

(e) with costs of collection or an attorney's fee, in case payment shall not be made at maturity.