What do the letter codes in Oracle user_contraints table's constraint_type column stand for?
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 ColumnO Read Only on a view ObjectP Primary Key ObjectR Referential (Foreign Key) ColumnU Unique Key ColumnV Check Option on a view Object