Database Software

Hi folks,
I have a doubt with arc_tolerance parameter. I'm using Spatial (9.2.0.4) and geodetic data (SRID 8292).
I'm using SDO_BUFFER and SDO_RELATE to retrieve some data from database. I read the manual that I need to use arc_tolerance with geodetic data and I can to specify the unit parameters in SDO_BUFFER function, the doubt is: what is the relationship between arc_tolerance and unit parameter in SDO_BUFFER function? What is the base for arc_tolerance? The arc_tolerance is related with unit parameter? I understood the SDO_ARC_DENSIFY explanation in usage notes about the usability for arc_tolerance.
The query that I'm using:
SELECT aCP.COD_GID, aCP.SHP_ID, aCP.SHP_GRIDCODE FROM TWGS_SHP_206 aCP WHERE SDO_RELATE (aCP.GEO_COORDENADAS, (SELECT SDO_GEOM.SDO_BUFFER(aFP.GEO_COORDENADAS, 100, 0.0005, 'units=km arc_tolerance=1') FROM TWGS_SHP_209 aFP WHERE aFP.COD_GID = 5), 'mask=ANYINTERACT querytype=WINDOW') = 'TRUE'

The strange in this query is if I change the arc_tolerance with the same value than tolerance the Oracle return an error:
ERROR in line 11:
ORA-29902: error in executing ODCIIndexStart() routine
ORA-13292: Incorrect ARC_TOLERANCE specification
ORA-06512: at "MDSYS.SDO_3GL", line 439
ORA-06512: at "MDSYS.SDO_GEOM", line 3065
ORA-06512: at "MDSYS.SDO_GEOM", line 3079

But if I execute separately the function SDO_BUFFER with arc_tolerance as the same value than tolerance I receive a corrext resul (a buffered geometry).
Any hints about this two problems? The arc_tolerance unit and the error above.
Thanks.