SAP SECURITY INTERVIEW - What is the difference between USOBX_C and USOBT_C

What is the difference between USOBX_C and USOBT_C?  

The table USOBX_C defines which authorization checks are to be performed within a transaction and which not (despite authority- check command programmed ). This table also determines which authorization checks are maintained in the Profile Generator.   The table USOBT_C  defines for each transaction and for each authorization object which default values an authorization created from the authorization object should have in the Profile Generator.