Using CLEAR one can set the retention policy to default and using NONE have no retention policy.
The example below shows the difference between the two.

——— Display retention policy and shows using the option CLEAR ———-
RMAN> show retention policy;

RMAN configuration parameters are:
CONFIGURE RETENTION POLICY TO REDUNDANCY 1; # default

RMAN> configure retention policy to redundancy 3;

new RMAN configuration parameters:
CONFIGURE RETENTION POLICY TO REDUNDANCY 3;
new RMAN configuration parameters are successfully stored

RMAN> show retention policy;

RMAN configuration parameters are:
CONFIGURE RETENTION POLICY TO REDUNDANCY 3;

-- Clearing the retention value set the redundancy to 1
RMAN> configure retention policy clear;

old RMAN configuration parameters:
CONFIGURE RETENTION POLICY TO REDUNDANCY 3;
RMAN configuration parameters are successfully reset to default value

RMAN> show retention policy;

RMAN configuration parameters are:
CONFIGURE RETENTION POLICY TO REDUNDANCY 1; # default

——— Display retention policy and shows using the option NONE ———-
RMAN> show retention policy;

RMAN configuration parameters are: Setting to NONE is not the same as clearing retention, it is disables it and commands like REPORT OBSOLETE or DELETE OBSOLETE reports error “RMAN-06525: RMAN retention policy is set to none”
RMAN> configure retention policy to none;

new RMAN configuration parameters:
CONFIGURE RETENTION POLICY TO NONE;
new RMAN configuration parameters are successfully stored

RMAN> show retention policy;

RMAN configuration parameters are:
CONFIGURE RETENTION POLICY TO NONE;

RMAN> configure retention policy clear;

old RMAN configuration parameters:
CONFIGURE RETENTION POLICY TO NONE;
RMAN configuration parameters are successfully reset to default value

RMAN> show retention policy;

RMAN configuration parameters are:
CONFIGURE RETENTION POLICY TO REDUNDANCY 1; # default

• Backup & Recovery
• RMAN