Lookup fields in Layer retrieve information from related categories.

Silvia Lee
Lookups are now part of Relationships. You can show information from a related category directly in your current one.
To display a value from a related category
Go to Edit Fields in your category.

The Edit Fields entry point for adding a lookup that pulls values from a related category.
Click the Create Field button and scroll to Relationships.
Choose the existing relationship you want to use.
Select the property you want to display (for example, Created By, Description, Drawing Location, or Updated At).

Choosing which property from a related category should be looked up and displayed.
Give your new field a name, then click Create Field. The value will appear automatically for each related item.

A configured lookup field surfacing related-category values directly in the current category.
Note, Lookups are now a part of Layer Formulas. You can alternately use the Formula Field to lookup values.
