Documentation
Connected Lookup Column
Connected Lookup column is custom column type that adds advanced functionality beyond the limitations of the built-in lookup field, such as filtering according to the value of another, master column in the same list item, filtering values according to a view and adding new values directly from the column.
The field can function as a stand-alone lookup column or as a master-detail cascading lookup column. When operating in the latter mode the column presents a selection of possible values from the source list, filtered by the master column. For example, attached to a complete list of cities, the column will only show cities located in the country, which value was beforehand selected in the Country column. When the Country field selection is changed, the corresponding values are fetched using AJAX technology, not requiring page refreshes.
The field can be used as a regular list column or as a site column. Site columns can be re-used across the site collection in the site where it was defined and all the subsites. It can be added to a list directly or added to a content type to be reused. Please note that both the master lookup column and the master-detail lookup column must be present in the same content type/list.
The Connected Lookup column can be attached to master column with both single and multiple selection, and also be configured to allow single or multiple selection of values.
NOTE: custom fields in SharePoint will remain read-only in Spreadsheet view and in Document Information Panel in Office 2007-2016.
