Hi,
Retroactive accounting:
A payroll run is repeated for a period for which payroll accounting has already been performed in the payroll past. Retroactive accounting is triggered during the payroll run for the current period if certain master and time data affecting the payroll past has been changed in the meantime.
Only changes to master and time data are relevant for retroactive accounting since previous payroll results must be corrected. A retroactive accounting limit can be set for the whole company and/or for just one employee.
Earliest Retroactive accounting
The payroll period furthest back in the past for which it is still possible to perform a retroactive accounting run.
It is stored in the HR control record for the payroll area.
Ceck the below link"
http://scn.sap.com/docs/DOC-35341
Thanks
Siva