FORWARD PROCESIING IN SAP

hi all experts,

I want to know when we doing backward scheduling and no-material is available ,then first system determine material availability date (is this date is in future or past of date which is determined in backward scheduling).take following example;

i am making order on 1st Oct2010,customer request for 15 oct.2010.transit time=1day,loading time=1day,transportation lead is=2days,pick\pack time=1day,then from backward scheduling system check for availability on 15-1-1-2=11oct2010,no material available, then system do forward scheduling how system propose new availability date ? then is there is any chance of availability before 11 ? system always propose a new availability date in future of that date which is determine in backward scheduling ?

ANOTHER my doubt is when we use MRP,REPLENISHMENT LEAD TIME IN backward OR forward scheduling ?

in what condition we use above ?

MANU TALWAR