When a person sends a letter to another individual within the same country and expects a reply from the receiver, he/she can enclose a self-addressed envelope using which the receiver would send a reply. An IRC is an equivalent of this concept that is used in case of international posts.
When a sender expects a reply for his/her international letter, and is unsure about the postage required for a reply, he/she can send an IRC to the receiver. The receiver can use the coupon and exchange it for adequate postage stamps from Universal Postal Union’s (UPU) postal services. The UPU is an agency of the UN that coordinates postal policies among its global members. Most constituent countries of the UN are members of the UPU.