Commanding Officers are the Nintendo Wars characters in charge of their respective armies. While they actually have no gameplay role and vary from map to map in Famicom Wars, and aren't around in the Game Boy Wars series (despite the presensce of a superior in Game Boy Wars 3), in Super Famicom Wars and the Advance Wars games, each CO affects their army's abilities.
Day-To-Day is the CO abilities that take effect daily.
Super Famicom Wars
COs can be selected for a match or a single-player file. On a single-player file, COs can't be changed at any point, including inbetween maps. Human player and 4-player matches are unaffected because they don't save anyway.
All computer-controlled COs have varied AI. Only Caroline, Billy Gates, and Yamamoto have any abilities that aren't involving AI, all Day-To-Day. All three have no weakness, and the latter two can be considered broken because of how powerful their Day-To-Day is.
The Super Famicom Wars COs have yet to make another appearance.
All COs except Andy have any Day-To-Day, but each Day-To-Day also includes a weakness, with the exception of Nell's, but she is unlocked only by beating Hard Campaign. Each Day-To-Day reflects the CO's own theme.
Each CO has a special ability called a CO Power, typically based on their strengths and usually giving them compensation on maps where their weaknesses are more punishing. The CO Power has to be charged up into a CO Meter, which is done by inflicting damage or recieving it, the latter charging up the CO Power four times as fast. How fast the meter is filled depends on the damage dealt or taken in cost amount. The CO Meter's amount increases by 20% of the base amount each time the CO Power is used, until the 10th CO Power usage at which point it is permanently reset to 200% of the base instead of 300%.
Advance Wars 2
In addition to a CO Power, each CO now has a Super CO Power, which is more effective than the CO Power but is also more expensive to use. The CO Meter is measured by stars. The Super CO Power can only be used when all of the stars are full, but the CO Power can be used any time all of the smaller stars are full, and doesn't necessarily completely empty the meter.
The CO charging times have also changed; damage taken value is now half the amount it was in AW1, so it's only double the damage dealt value. Each CO Meter star is worth 18000G of damage dealt (or 9000G of damage taken, or a combination of the two) initially. The Star Value goes up by 3600G of the base amount each time either CO Power is used, until the 10th CO Power usage at which point it is permanently reset to 36000G of damage dealt instead of 54000G.