rman duplicate

Hi All,

env: 10.2.0.4 rdbms / ASM  - Prod and it needs to be cloned to a different host

Could you tell me how (or where from)  rman derives set until scn value while composing memory script?

Example

Starting Duplicate Db at 14-feb-2013 05:54:54

contents of Memory Script:
{
   set until scn  12460556894825;
   set newname for clone datafile  1 to new;
   set newname for clone datafile  2 to new;
   set newname for clone datafile  3 to new;
   set newname for clone datafile  4 to new;
   set newname for clone datafile  5 to new;
   set newname for clone datafile  6 to new;
   set newname for clone datafile  7 to new;
   set newname for clone datafile  8 to new;