The Office 365 Virtual Journal feature brings with it the possibility to migrate large journal archives to Office 365, using multiple target mailbox/personal-archives and specifying criteria such as number of items and size of target. This article explains more detail about this splitting process.
There are two ways that data can be split:
Number of Items
This is the simplest form. The splitting to a new mailbox/personal-archive occurs when the specified number of items are routed to the target mailbox or personal archive.
Size of Items
The splitting here is determined by the size of the items from the source environment. In the case where migration is from Enterprise Vault, the size used is the larger of the original size versus compressed size. When the amount of data routed to a target container reaches the specified value, a new target container will be created.