Additional clarifications from Trion (Elrar posted):

"It's also important to note that while one shard might be open for Guardians to transfer to, it may not have any room for Defiants. So what a "select shard" means can also depend on which faction you play."

One thing I've noticed is that they haven't said anything about restricting the source shard & faction. So at this point it seems that everyone will be able to transfer, no matter what faction & shard you're currently on (as long as the destination shard & faction is on the list).