Explanation:

Use the Update-MailboxDatabaseCopy cmdlet to seed or reseed a mailbox database copy.
We reseed the database with a new copy by issuing the following command.
[PS] C:\>Update-MailboxDatabaseCopy -Identity "DB1\EX2"
For long reseeds where you do not want to leave your Exchange Management Shell open, or when scripting a reseed and you don't want the script to have to wait for the reseed to complete, you can use the -BeginSeed parameter.
[PS] C:\>Update-MailboxDatabaseCopy "-Identity "DB1\EX2" -BeginSeed
The BeginSeed parameter is useful for scripting reseeds, because with this parameter, the task asynchronously starts the seeding operation and then exits the cmdlet.
Incorrect:
* The MaximumSeedsInParallel parameter is used with the Server parameter to specify the maximum number of parallel seeding operations that should occur across the specified server during a full server reseed operation. The default value is 10.