In a stack system, if only one switch's ESN is used to apply for a license, the license takes effect in the entire system after being activated and does not affect the stack upgrade or active/standby switchover. If the stack system is split, the switch whose ESN is used for license application is not affected. However, new configurations related to licensed features cannot be added on the other switches in the stack. To prevent this issue, you are advised to apply for a license for all the switches in a stack system.