Attached data is a custom field assigned to atoms or brackets. It has an identifier string (name) and a value. Furthermore, a query operator can describe different restrictions in queries. The dialog provides an interface to set the properties of such a field.
For more information about the attached data, see the Query Guide. Attaching data in MarvinSketch is described in Attached Data. The Attach Data dialog is customizable; configuration options and a small example can be found in Configure the Attach Data Dialog.