Xtype and type differences in SQL Server system table sysobjects

Sysobjects a row in the table for each object created within the database (constraints, defaults, logs, rules, stored procedures, and so on). Only in tempdb, each temporary object occupies a row in the table. There are two fields in sysobjects that represent the object type (Type,xtype).
The type is available in SQL Server 6.0, and xtype appears in SQL Server 7.0.
Comparing the type with the xtype, there are some differences: type:
K = PRIMARY KEY or UNIQUE constraint
R = Rule
Xtype
PK = PRIMARY KEY constraint (type is K)
UQ = UNIQUE constraint (type is K)

Xtype type
-the same
c = Check constraint c = CHECK constraint
d = defaults or default constraints D = defaults or DEFAULT constraints
f = FOREIGN key constraint f = FOREIGN key constraint
fn = Scalar function fn = scalar function
if = inline table function if = inline table function
p = Stored Procedure p = stored Procedure
RF = replication filter stored Procedure RF = Copy Filter stored procedure
s = system table S = System table
tf = table function tf = table function
TR = Trigger TR = Trigger
u = User table U = User table
v = View V = view
x = extended Stored Procedure x = Extended stored Procedure
L = Log L = Log

-A different
PK = PRIMARY key constraint (type K) k = PRIMARY key or UNIQUE constraint
UQ = UNIQUE constraint (type is K) R = Rule

Xtype and type differences in SQL Server system table sysobjects