The last coookie counts principle is a rule applied by affiliate networks and merchants which manage themselves their affiliate programs.
According to the last cookie counts principle, when two or more affiliates are involved in a sale, only the last affiliate in touch with the consumer (by way of a click) is given credit for the conversion.
For instance, if a consumer discovers and chooses a product by clicking on a content affiliate’s link and then uses an affiliate voucher code site before buying, the commission is given to the voucher code site. When the consumer uses the affiliate link on the voucher code site, the cookie which is initially tied to the first affiliate ID is overwritten.
For more details, see cookie overwriting.