boris,

Of course UNDO didn't need undo data from redo logs. UNDO tbs is not like TEMP tbs that loss TEMPorary data after instance is aborted. So all UNDO data to be played back are all in it. But what I meant with "UNDO INFORMATION" is the corresponding SCN, is recorded in the REDO LOGS. Read my posts here in this thread.

My assumption is that, for example, a long transaction during its first execution, its corresponding SCN is recorded to the CURRENT REDO LOGS(let's say SEQ 50), and after the transaction completed before commit, REDO LOG finished with sequence 60. REDO LOG SEQ 50, have to be read during transaction recovery. But it seems like:
after a REDO LOG is archived the SCN number that is UNCOMMITTED will be passed to the new CURRENT online REDO LOG. So that, during INSTANCE recovery the only REDO LOGS that are read are CURRENT, ACTIVE(ONLINE).

So, I could be wrong with ARCHIVE LOG being read during INSTANCE RECOVERY, but I still believe that ONLINE REDO LOG is used.