SharePoint does not have a built-in method of creating connected lookup fields (e.g. if I select a specific List Item, select another column only from that List Item). Connected Lookups provides this ability. You can have a parent with multiple children, and so far is the only working solution I’ve found that can have multiple children.
In this example, the Parent List has three columns: Title, Child1, Child2.
The Child List has four columns: Title, ParentDrop (mapping to Title), Child1 (mapping to Child1), and Child2 (mapping to Child2). ParentDrop has a Parent Column of “(None)”, and the Broadcast flag checked. Child1 has a Parent Column of “Title” (from our Parent List), and the Broadcast flag checked. Child2 has a Parent Column of “Title”, and the Broadcast flag is unchecked. Here is the example:
Step 1: Selecting a value for ParentDrop:
Step 2: There is only one value to select for Child1 once we select our ParentDrop value:
Step 3: Again, Child2 only has one value based on the value from Child1: