Undo Retention Gurantee.

When using AUM, you have three kind of undo extents

1)ACTIVE This means that undo extent is Active and being currently used by a transaction

2)UNEXPIRED This represents the extents which are required to satisfy the time specified by Undo_retention Initialisation parameter.

3)EXPIREDThese are extents which are not being used by transaction and have crossed the time specified by Undo_retention .So generally when undo segment space is full then we try to use the Expired extents and if there is no space to either get a new extent or reuse expired extents, then we go and use Unexpired extents. This sometimes lead to ORA-1555 error.

Starting from 10g, a new feature called Retention Guarantee has been introduced. If Retention Guarantee is enabled then you can be assured that you will not be reusing the Unexpired extents but this will mean higher chances of getting ORA-30036 error i.e failing to extend Undo Rollback segmentPlease refer following post for more details