All flights need to be AY (Finnair) flights. I'm not really sure, but I suppose this means AY metal = codeshares are excluded.

You can book several tickets with one voucher, either in one booking for several people or by making several bookings. The unused portion of the voucher will remain valid, you just enter the codes again for the next booking.

You will lose the cents in the process, ie if you buy a ticket that costs €399.01, 1000.00 will remain on the voucher, not 1000.99.