Send on Behalf permission

I am wondering why I can't see GrantSendOnBehalfTo permission with Get-MailboxPermission or Get-ADPermission?

I can find FullAccess and SendAs with Get-MailboxPermission or Get-ADPermission. But I must use Get-Mailbox user | fl GrantSendOnBehalfTo to view Send on Behalf permission.

Why is this?

Thanks,

Paul