ARM uses the coverage index (how much coverage an access point can see from its own access points) to determine if power on an access point should be increased. This was a bigger issue back in the days, when most deployments were for coverage instead of density. These days, there is so much density in deployments, access points will see that there is enough coverage and do nothing. If you have a sparse deployment and an access point is not limited by its max TX power, it will raise the power if another access point is out of service and it determines it can increase the coverage.