"ORA-1715 : UNIQUE may not be used with a cluster index" but why?

Question

"ORA-1715 : UNIQUE may not be used with a cluster index" but why and what "may" means here? Any comments will be welcomed, thank you and best regards;
show rel
release 1002000300
CREATE CLUSTER sc_srvr_id (
srvr_id NUMBER(10)) SIZE 1024;
SELECT cluster_name, tablespace_name, hashkeys,
degree, single_table FROM user_clusters;
CREATE UNIQUE INDEX idx_sc_srvr_id ON CLUSTER sc_srvr_id;
ERROR at line 1:
ORA-01715: UNIQUE may not be used with a cluster index
CREATE INDEX idx_sc_srvr_id ON CLUSTER sc_srvr_id;
SELECT index_name, index_type, tablespace_name
FROM user_indexes where index_name like '%SRVR%' ;
CREATE TABLE cservers (
srvr_id    NUMBER(10),
network_id NUMBER(10),
status     VARCHAR2(1),
latitude   FLOAT(20),
longitude  FLOAT(20),
netaddress VARCHAR2(15))
CLUSTER sc_srvr_id (srvr_id);
ALTER TABLE cservers add constraint pk_srvr_id primary key (srvr_id ) ;
SELECT index_name, index_type, tablespace_name
FROM user_indexes where index_name like '%SRVR%' ;
INDEX_NAME                     INDEX_TYPE
TABLESPACE_NAME
IDX_SC_SRVR_ID                 CLUSTER
USERS
PK_SRVR_ID                     NORMAL
USERSdo we really need another pkey index here?

Answer 1

"May" has different meanings, one of which is:
(used to express opportunity or permission)
Metalink note 19067.1 says:
This is not permitted.
... which agrees with the above meaning of it.
Besides these, it does not make any sense to me to create a unique index on a cluster. You can have primary keys in the tables you include in the cluster, it depends on your business requirement. But why do you need a unique index on a cluster?