Anyone familiar with the different types of upgrades given and their priority? On a recent flight I was trying to use an upgrade certificate (one class upgrade from J to F) and was told the last seat was taken as a C.I.S upgrade and they had priority over the certificate? Anyone know what CIS stands for?

Found it very strange that a free upgrade was given ahead of a voucher which would be similar to using a mileage upgrade?

What are the different upgrade types & priorities given?