Help explain character_set_database and collation_database

[Gary Yang]

Hello! Would you please help me understand these two variables? The reference manual reads:

"The character set and collation for the default database can be determined from the values of the character_set_database and collation_database system variables. The server sets these variables whenever the default database changes."

Suppose I issued SQL statement "use db1;". Now db1 is my default database. If db1 was created with character set A and collation B, which are different from the values of character_set_database (X) and collation_database (Y), how could these two variables affect my "default database" db1? Thank you in advance.