What is the difference between C (Check) and U (Unmentioned)? | SAP Security interview questions

Background:
When defining authorizations using Profile Generator, the table USOBX_C defines which authorization checks should occur within a transaction and which authorization checks should be maintained in the PG. You determine the authorization checks that can be maintained in the PG using Check Indicators. It is a Check Table for Table USOBT_C.
In USOBX_C there are 4 Check Indicators:
=> CM (Check/Maintain)
-An authority check is carried out against this object.
-The PG creates an authorization for this object and field values are displayed for changing.
-Default values for this authorization can be maintained.
=> C (Check)
-An authority check is carried out against this object.
-The PG does not create an authorization for this object, so field values are not displayed.
-No default values can be maintained for this authorization.
=> N (No check)
-The authority check against this object is disabled.
-The PG does not create an authorization for this object, so field values are not displayed.
-No default values can be maintained for this authorization.
=> U (Unmaintained)
-No check indicator is set.
-An authority check is always carried out against this object.
-The PG does not create an authorization for this object, so field values are not displayed.
-No default values can be maintained for this authorization..