Difference between "usermod -aG" and "usermod -G" options in linux?

[Brief-Wishbone]

I'm on centos 7. I'm wondering what could be the differences between these two?

I've read this but could not grasp what they're trying to say. Can you give me an example scenario to work with?

https://unix.stackexchange.com/questions/608822/difference-between-usermod-ag-and-usermod-g-options

 

[Guest]

the "-a" in usermod is append and "-G" assigns supplementary groups. https://linux.die.net/man/8/usermod

 

So lets say you have a user named joe and its in groups joe,storage,power. joe would be the primary group of joe.

 

usermod -G wheel joe

joe now belongs to joe,wheel

 

usermod -aG wheel joe

joe now belongs to joe,storage,power,wheel