I suppose it might be reserved for temp files.

HELLODBA.COM>show parameter db_files

NAME                                 TYPE                             VALUE
------------------------------------ -------------------------------- -----------
db_files                             integer                          200

HELLODBA.COM>select file_id, file_name, tablespace_name, relative_fno from dba_data_files;

   FILE_ID FILE_NAME                                          TABLESPACE_NAME                RELATIVE_FNO
---------- -------------------------------------------------- ------------------------------ ------------
         4 C:\ORACLE11GR2\ORADATA\ORA11R2\USERS01.DBF         USERS                                     4
         3 C:\ORACLE11GR2\ORADATA\ORA11R2\UNDOTBS01.DBF       UNDOTBS1                                  3
         2 C:\ORACLE11GR2\ORADATA\ORA11R2\SYSAUX01.DBF        SYSAUX                                    2
         1 C:\ORACLE11GR2\ORADATA\ORA11R2\SYSTEM01.DBF        SYSTEM                                    1
         5 C:\ORACLE11GR2\ORADATA\ORA11R2\DEMO_DATA.DBF       DEMO_DATA                                 5
         6 C:\ORACLE11GR2\ORADATA\ORA11R2\DEMO_INDX.DBF       DEMO_INDX                                 6
         7 C:\ORACLE11GR2\ORADATA\ORA11R2\DFILE1.DAT          MULTIFILE_DATA                            7
         8 C:\ORACLE11GR2\ORADATA\ORA11R2\DFILE2.DAT          MULTIFILE_DATA                            8
         9 C:\ORACLE11GR2\ORADATA\ORA11R2\DFILE3.DAT          MULTIFILE_DATA                            9
        10 C:\ORACLE11GR2\ORADATA\ORA11R2\DFILE4.DAT          MULTIFILE_DATA                           10
        11 C:\ORACLE11GR2\ORADATA\ORA11R2\DFILE5.DAT          MULTIFILE_DATA                           11
        12 C:\ORACLE11GR2\ORADATA\ORA11R2\DFILE6.DAT          MULTIFILE_DATA                           12

12 rows selected.

HELLODBA.COM>select file_id, file_name, tablespace_name, relative_fno from dba_temp_files;

   FILE_ID FILE_NAME                                          TABLESPACE_NAME                RELATIVE_FNO
---------- -------------------------------------------------- ------------------------------ ------------
         1 C:\ORACLE11GR2\ORADATA\ORA11R2\TEMP01.DBF          TEMP                                      1

While, in a 10046 trace file of disk sorting, we can find the absolute file number of temp files is RELATIVE_FNO+DB_FIELS

WAIT #5: nam='direct path write temp' ela= 23472 file number=201 first dba=17398 block cnt=7 obj#=74177 tim=1800385730788
WAIT #5: nam='direct path write temp' ela= 11994 file number=201 first dba=17405 block cnt=7 obj#=74177 tim=1800385743176
WAIT #5: nam='direct path write temp' ela= 605 file number=201 first dba=17412 block cnt=7 obj#=74177 tim=1800385744045

and from the fixed table (abstract from dba_temp_files) point to temp files, can find its absolute file number.

HELLODBA.COM>select addr, tfnum, tfafn from sys.x$kcctf tf;

ADDR          TFNUM      TFAFN
-------- ---------- ----------
0DB34304          1        201

Regards,
Fuyuncat