how does the migrationbatch decide on mailbox allocation?

new-migrationbatch -local -name blah -csvdata ([System.IO.File]::ReadAllBytes("c:\dump\blah.csv")) -Targetdatabases db1,db2, db3 -autostart -notificationemails me@msn.com -autoretrycount 2

also is it necessary to  remove-migrationbatch ?

Hi,

Whats you means about mailbox allocation?
It will move mailbox one by one basic on the list in .csv file. If error occur, it will skip and begin next.

If you want to stop MigrationBatch, we can run Stop-MigrationBatch Identity Batch name, Remove-MigrationBatch is used for delete batch which is completed.
Heres a blog about Performing Local Moves using MigrationBatch in Exchange 2013, for your reference:
http://blogs.technet.com/b/localmovesusingmigrationbatchine15/archive/2014/05/22/performing-local-moves-using-migrationbatch-in-exchange-2013.aspx

Thanks

I have multiple databases under exchange 2013, using the -Targetdatabases , you can tell it your databases names (Targetdatabases db1,db2, db3 ) and it will automatically spread out your users to these databases.

I was wondering how it decided or if it just goes down the list (csv)