RoW means Rest of World. SASE, means Self-addressed Stamped Envelope. A Self-addressed Envelope is an envelope on which you write your own address and then send to someone so that when they reply to you they can use your envelope and save themselves time and expense. If you live outside of the country to which you are sending the SASE the stamp will not be honoured by the foreign post office. This is the same for Canadians.
Canadian stamps are for mailing letters inside Canada.
UK stamps are for mailing letters inside the UK, etc. When the person in the other country receives the IRC they can take it to their local post office and be given one first class postage stamp of that county. In remote areas of the US and Canada one may need to order the IRC's from the post office and wait for them to arrive. An IRC is much more expensive than a stamp. Each one may cost several dollars or pounds.