When the general reaches LVL 60, you will be able to upgrade its skill for one level only, and when reaching LVL 70, you will be able to upgrade it to another level.
And every time you upgrade an active skill, certain additional and generous characteristics will be added in addition to the skill itself, and it will be calculated as (permanent skill). It will be used when the general is activated only, and not necessarily to activate the ultimate skill; they are separated.