What is the difference between installing target dirsync agent when SIDHistory account is populated vs where its not? (4370456)

What is the difference between installing target dirsync agent when SIDHistory account is populated vs where its not?

When target dirsync agent is installed, there's a dialog, asking to provide source account for SIDHistory migration purposes. It can also be left unpopulated. Is it mandatory to populate this account and what are implications of filling this dialog?

Clarification of ODM AD documents with some technical background

Target domain dirsync installation is using a target domain service account to perform domain-specific tasks, initiated by ODM AD. The field with source SIDHistory account credentials is affecting the way, that target dirsync agent is communicating with the source domain for SIDHistory migration purposes.
ODM AD is performing DsAddSidhistory API call, which is explained here:
Using DsAddSidHistory - Win32 apps | Microsoft Learn

There are mainly 2 ways, the dirsync agent is authenticating against the source domain:
1) SIDHistory account is populated during installation of dirsync agent. No trust is required during the migration period. Specified account should be a member of BUILTIN\Administrators security group in the source domain.
2) SIDHistory account is not populated when agent is installed. Established domain trust is mandatory during the migration period and specified target domain service account should be a member of BUILTIN\Administrators security group in the source domain.

Note: SIDHistory functionality is useless without established domain trust and used for target users to be able to access source domain resources over the trust.