Get-MoveRequestStatistics ShardBytesTransferred - What are ShardBytes?

[titlerequired]

Running a Hybrid Migration at the moment, Exchange 2016 to Exchange Online.

Completed several hundered moves without many issues other than Source/Target Principal issues.

One mailbox has shown an 'item not in target' so we removed the request, resynced and attempted to complete the migration again with the same issue.

On closer inspection, running Get-MoveRequestStatistics shows it has transferred nearly 18gb of data for a 3gb mailbox.

The difference in size is taken up by the 'ShardBytesTransferred' which totals just over 14gb.

Does anyone know what these are?

Is it simply where the request has had overhead/retries/miscellaneous issues?

[#2]

A SHARD mailbox is a hidden mailbox for user's who's mailboxes haven't been migrated to Exchange Online. There are services in M365 that require a cloud mailbox to store information (generally this is around collaboration), so a SHARD mailbox for the non-migrated user is created to store that information. When the migration of mailbox to EXO completes, the EXO based SHARD mailbox is then merged into the mailbox migrated from on-premises. If this is failing, then you will probably need to contact Microsoft support to fix it up