It depends on the source database. Different databases support the various system values in different ways and to different extents, as determined by the features offered by the database.
For the iSeries. &SEQNO will hold the journal sequence number (when mirroring). However it may not be unique as when a journal receiver is rolled, the sequence number can be reset back to 0 for the new receiver. To be pretty much assured of generating a unique identifier you should also use the transaction &TIMSTAMP.
For the zSeries, &SEQNO will be the LRSN or RBA when mirroring.
I do not believe that &SEQNO is supported by any other database.