What do the letter codes in Oracle user_contraints table's constraint_type column stand for?

select distinct constraint_type from user_constraints;

C
-
C
P
R
U

Type Code   Type Description  	Acts On Level
C   Check on a table 	Column
O   Read Only on a view 	Object
P   Primary Key 	Object
R   Referential AKA Foreign Key 	Column
U   Unique Key 	Column
V   Check Option on a view 	Object