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A Header/Detail form is created in much the same manner as a Standard form. The primary differences are 1) there is an additional table for the detail section of the form and 2) the configuration of the detail section is done using a special kind of control.
Definition of the detail table is described here: Detail Table.
When the form is defined as Header/Detail, an additional control type named “Form Detail Section” becomes available.
Add a row to hold the Form Detail Section control. This row is typically 1 x 1 or 1 x 2. The column holding the Form Detail Section control should be wide enough to present all the columns of the detail section in a reasonable fashion.
Drag/drop the Form Detail Section control to the desired row and column in the form.
Enter the number of columns to be used in the detail section.

The number of columns must include one for the linking value, even if it will not be displayed in the grid.
The columns for the grid are displayed in the control.

The columns are displayed across the entire column when presented to the user, even if it doesn’t appear so in the layout.
Drag/drop the desired columns from the Database Fields (Detail) list to the desired position in the row.
The linking value field must be included in the grid fields.
All functions such as formulas, default values, etc. described for header section controls apply to grid controls as well unless otherwise noted.