@EiC4733 If you are trying to limit the lookup column options from parent list to child list in dropdown on child list form, this is not possible using SharePoint out of the box capabilities.
If you are using SharePoint online modern experience, you can customize list form using Power apps to achieve your requirements OR use calculated column formula approach (depends on your requirements, it might not work) mentioned in below link.
Similar thread: How to filter lookup column based on another column from using specific criteria?
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