dba_role_privs.DEFAULT_ROLE show which ROLES is DEFAULT for users.
by default all grant all is default for user but when you explicit specify DEFAULT ROLE for user then only specify role is DEFAULT role for user.
consider.
SQL> select grantee, granted_role , default_role from dba_role_privs
  2  where grantee = 'SCOTT';

GRANTEE                        GRANTED_ROLE                   DEF
------------------------------ ------------------------------ ---
SCOTT                          DBA                            YES
SCOTT                          DEF                            YES
SCOTT                          CONNECT                        YES
SCOTT                          RESOURCE                       YES

SQL> alter user SCOTT DEFAULT ROLE resource;

User altered.

SQL> select grantee, granted_role , default_role from dba_role_privs
  2  where grantee = 'SCOTT';

GRANTEE                        GRANTED_ROLE                   DEF
------------------------------ ------------------------------ ---
SCOTT                          DBA                            NO
SCOTT                          DEF                            NO
SCOTT                          CONNECT                        NO
SCOTT                          RESOURCE                       YES

SQL> alter user SCOTT DEFAULT ROLE def;

User altered.

SQL> select grantee, granted_role , default_role from dba_role_privs
  2  where grantee = 'SCOTT';

GRANTEE                        GRANTED_ROLE                   DEF
------------------------------ ------------------------------ ---
SCOTT                          DBA                            NO
SCOTT                          DEF                            YES
SCOTT                          CONNECT                        NO
SCOTT                          RESOURCE                       NO

SQL>