Managing database availability groups

Hello,

I have general Question For DAG in exchange server I am following this link https://technet.microsoft.com/en-us/library/dd638129(v=exchg.150).aspx 

my questions is

1) -WitnessDirectory C:\DAGWitness\DAG1.contoso.com   does Witness Directory need to be Shared?? DAG1.contoso.com is folder XD ???

2) for the add I know he add the database with delay for 3 days can some one explain the 2 suspend and update command in simple words ??

Add-MailboxDatabaseCopy -Identity DB1 -MailboxServer MBX3 -ReplayLagTime 3.00:00:00 -SeedingPostponed
Suspend-MailboxDatabaseCopy -Identity DB1\MBX3 -SuspendComment "Seed from MBX4" -Confirm:$False
Update-MailboxDatabaseCopy -Identity DB1\MBX3 -SourceServer MBX4
Suspend-MailboxDatabaseCopy -Identity DB1\MBX3 -ActivationOnly

I know I am asking too much but I am new at Exchange.

Regards,

Ayesh.