Hi,
A Foreign Security Principal (FSP) is an object created by the system to represent a security principal in a trusted external forest. These objects are created in the Foreign Security Principals container of the domain. They can be added to domain local security groups and granted permissions.
For example: When adding a user from domain A for the first time to a group from domain B from another forest, this creates an FSP in domain B and adds this FSP to the group from domain B.
For your reference:
https://social.technet.microsoft.com/wiki/contents/articles/51367.active-directory-foreign-security-principals-and-special-identities.aspx
If there orphaned Foreign Security Principals in the container, you can clean up them through ADUC,ADSI or PowerShell command.
For your reference:
https://4sysops.com/archives/clean-up-orphaned-foreign-security-principals/(Third-party link)
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Best Regards,