In other words, how to add a choice field to sharepoint list that has values depending on the current selection?
This kind of requirement is wildely asked in various forums. By build such parent child relationship you can simplify SharePoint Lookup very easily.
For instance, you can conduct cross-site lookup, filter lookup, connected lookup and so on.
To realize such requirement you need take a look at SharePoint Cascaded Lookup.
Blow are the features of SharePoint Cascaded Lookup:
1. Cross-site look up SharePoint lists in other sites
2. Build Parent-Child Relationship by filtering SharePoint columns,
3. Filter looked-up column by Filter Column in the looked-up list,
4. Filter looked-up column by Parent Column in the Cascaded Lookup Column residing list,
5. Two-way conversion to default SharePoint Lookup Column type or SharePoint Cross-Site Lookup Column type
6. Support filtering multiple-values Cascaded Lookup Column.
More information please visit SharePoint Cascaded Lookup.