Glassfish JDBC: Do I have to use only jdbc/__default?

Glassfish JDBC: Do I have to use only jdbc/__default?

I try to use Glassfish/MySQL. I have created JDBC resource and JDBC
connection pool for MySQL.
But if I tried to put MySQL JDBC resource in , nothing works.
Then if I tried to modify jdbc/__default and change its connection pool
from DerbyPool to MySQL, it works. My entity gets persists to the correct
table.
So do I have to use jdbc/__default only as my JDBC resource for my app?
How can I use the JDBC resource and JDBC connection pool I created in my
app?
I really have hard time understanding how to use JDBC in Glassfish.
This is my first time to ask question in this forum. Thank you very much.