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

Seems P means primary key and R means foreign key, correct? What are U and C?

From the 12cR1 documentation:

C - Check constraint on a table  
P - Primary key  
U - Unique key  
R - Referential integrity  
V - With check option, on a view  
O - With read only, on a view  
H - Hash expression  
F - Constraint that involves a REF column  
S - Supplemental logging

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