Your migration will stop with this status once the maximum data transfer limit of the license selected has been reached. If you know that a mailbox is larger than 50GB, you can increase the number of licenses allowed per mailbox. See the article How do I enable consumption of multiple licenses for a single item?
Example: If you migrate a mailbox with a mailbox migration license and the migration reaches 50GB of data transferred, the migration will stop and show the status of Transfer Limit Reached. You will need to purchase another mailbox migration license (if you do not have an extra mailbox licenses in your account) and submit the mailbox migration again to continue from where it left off.