With the arc form type setting, the building type of the selected hall in the hall settings dialog is changed. The roof of the hall has become to have a vaulted roof geometry.
Location of Arc Form Type
Select the hall and click the right mouse button. Click the Properties line from the right click menu that opens.
Enter the hall settings.
Select the hall you want to change the roof form for from the Hall list.
Check the arc form option in the form type section.
The hall you choose will be formed with the arc form type.
View of hall 1 triangular, hall 2 arc form and hall 3 with rectangular type selected