Relating child records to parent records during import
Key Takeaway
Import parent records first. Each child row then needs the parent's Salesforce ID, or another value a tool can use to find the parent.
When you import child records, such as contacts that belong to accounts, each child row needs a value that points to its parent record so Salesforce can fill in the relationship field. You can give the parent's Salesforce ID directly, or you can use a tool feature that finds the parent for you from another value, such as an external ID or a name. The order you import data in, and how you sort it, also affect whether the import succeeds.
How a child record links to its parent
A lookup is a relationship between two objects, stored in a lookup field that links one record to another. The object that holds the lookup field is the source object, and the object it points to is the target object.
For example, a Contact has an Account lookup field (AccountId) that links the contact to its account. Contact holds the field, so it is the source object, and Account is the target object.
The child's relationship field needs the parent's Salesforce ID. For example, a contact's Account lookup requires the Account ID, not the account name.