When the snapmirror initialize command copies a volume, it creates a Snapshot copy of all the data on the source and transfers it to the destination. The destination is a volume that you have already created and marked restricted. After SnapMirror finishes transferring the data, it brings the destination online in a read-only state. This version of the destination is the baseline for the first incremental update.
While the initial data transfer is taking place, the destination is marked invalid in the output of a vol status command. The volume becomes valid and goes online after the initial transfer is complete.