Oracle dba_ind_partitions vs dba_segments misunderstanding

Question

I've written a piece of SQL script to move all indexes inside a tablespace to other one, the code is OK and working fine. but I am confused about partitioned indexes that why some of them are inside DBA_SEGMENTS and some of them are inside DBA_IND_PARTITIONS and why the ones are inside DBA_IND_PARTITIONS are not inside DBA_SEGMENTS.

Could someone explain the difference and usage of these tables in Oracle

This is my script to move indexes from one tablespace to other

DECLARE
  -- SETTING   
  SOURCE_TS VARCHAR2(30) := 'source_tablespace'; -- SOURCE TABLESPACE
  DEST_TS   VARCHAR2(30) := 'destination_tablespace'; --DESCTINATION TABLESPACE
  -- SHOULD_DROPINDEX           BOOLEAN := FALSE;
  ----------------------------------------------
  TOP                  NUMBER := 2;
  COUNTER              NUMBER := 0;
  SHOULD_MOVEINDEX     BOOLEAN := TRUE;
  PARALLEL_DEGREE      NUMBER := 4;
  ENABLE_PARALLEL      BOOLEAN := TRUE;
  INDEX_NAMES_TO_SCAPE VARCHAR2(1000) := '';
  ----------------------------------------------   
  -- VARIABLES   
  SQLCMD    VARCHAR2(1000);
  STARTTIME NUMBER;
BEGIN

  STARTTIME := DBMS_UTILITY.GET_TIME();

  DBMS_OUTPUT.PUT_LINE('STARTING TIME : ' ||
                       TO_CHAR(SYSDATE, 'YYYY/MM/DD HH24:MI:SS'));

  DBMS_OUTPUT.PUT_LINE('----------------------------------------------------');

  FOR IDXS IN (SELECT OWNER || '.' || INDEX_NAME FN,
                      INDEX_NAME IDN,
                      INDEX_TYPE IDT,
                      PARTITIONED,
                      (CASE
                        WHEN UNIQUENESS = 'UNIQUE' THEN
                         'Y'
                        ELSE
                         'N'
                      END) ISUNIQUE,
                      OWNER
                 FROM DBA_INDEXES
                WHERE (TABLESPACE_NAME = SOURCE_TS OR
                      INDEX_NAME IN
                      (SELECT DISTINCT SEGMENT_NAME
                          FROM DBA_SEGMENTS
                         WHERE TABLESPACE_NAME = SOURCE_TS) OR
                      INDEX_NAME IN
                      (SELECT DISTINCT INDEX_NAME
                          FROM DBA_IND_PARTITIONS
                         WHERE TABLESPACE_NAME = SOURCE_TS))
                ORDER BY OWNER ASC, INDEX_NAME ASC               
               ) LOOP

    IF COUNTER >= TOP THEN
      DBMS_OUTPUT.PUT_LINE('EXIT ON COUNTER EXCEED');
      EXIT;
    END IF;

    IF INSTR(INDEX_NAMES_TO_SCAPE, IDXS.IDN) >= 1 THEN
      CONTINUE;
    END IF;

    IF IDXS.PARTITIONED = 'YES' THEN

      FOR PIDX IN (SELECT *
                     FROM DBA_IND_PARTITIONS
                    WHERE INDEX_OWNER = IDXS.OWNER
                      AND INDEX_NAME = IDXS.IDN
                      AND TABLESPACE_NAME = SOURCE_TS) LOOP
        -- ** EXECUTE LOCAL (PARTITIONED) INDEX REBUILD COMMAND **
        SQLCMD := 'ALTER INDEX ' || IDXS.FN || ' REBUILD PARTITION ' ||
                  PIDX.PARTITION_NAME || ' TABLESPACE ' || DEST_TS;

        IF ENABLE_PARALLEL = TRUE THEN
          SQLCMD := SQLCMD || ' PARALLEL ' || PARALLEL_DEGREE;
        END IF;

        --SQLCMD := SQLCMD || ' NONLOGGING ';
        DBMS_OUTPUT.PUT_LINE(SQLCMD);
        EXECUTE IMMEDIATE SQLCMD;
      END LOOP;

      IF ENABLE_PARALLEL = TRUE THEN
        SQLCMD := 'ALTER INDEX ' || IDXS.FN || ' NOPARALLEL ';
        DBMS_OUTPUT.PUT_LINE(SQLCMD);
        EXECUTE IMMEDIATE SQLCMD;
      END IF;

    ELSE
      -- ** EXECUTE INDEX REBUILD COMMAND **
      SQLCMD := 'ALTER INDEX ' || IDXS.FN || ' REBUILD TABLESPACE ' ||
                DEST_TS;

      IF ENABLE_PARALLEL = TRUE THEN
        SQLCMD := SQLCMD || ' PARALLEL ' || PARALLEL_DEGREE;
      END IF;

      --SQLCMD := SQLCMD || ' NONLOGGING ';
      DBMS_OUTPUT.PUT_LINE(SQLCMD);
      EXECUTE IMMEDIATE SQLCMD;

      IF ENABLE_PARALLEL = TRUE THEN
        SQLCMD := 'ALTER INDEX ' || IDXS.FN || ' NOPARALLEL ';
        DBMS_OUTPUT.PUT_LINE(SQLCMD);
        EXECUTE IMMEDIATE SQLCMD;
      END IF;

    END IF;
    COUNTER := COUNTER + 1;
    DBMS_OUTPUT.PUT_LINE('#');
  END LOOP;
  DBMS_OUTPUT.PUT_LINE('----------------------------------------------------');

  DBMS_OUTPUT.PUT_LINE('END TIME : ' ||
                       TO_CHAR(SYSDATE, 'YYYY/MM/DD HH24:MI:SS'));
  DBMS_OUTPUT.PUT_LINE('DURATION: ' ||
                       TO_CHAR(DBMS_UTILITY.GET_TIME() - STARTTIME) ||
                       ' MS.');
EXCEPTION
  WHEN OTHERS THEN
    DBMS_OUTPUT.PUT_LINE('ERR MSG: ' || SQLERRM || ', CODE: ' || SQLCODE);
END;