The class behind the form section is SPF.Client.Forms.FormControls.SectionElement, which inherits from SPFGroupBox.
Similar to the wizard, when the client starts up it looks for all classes that inherit from the SectionElement, and it stores them in overridden sections collection. When a form is displayed in the client for each section it will look in the collection to find a matching overridden class using the following logic:
Dim lstrModifiedSectionName As String = pobjISPFSection.IObject.InternalName.Replace("-", "")
lstrModifiedSectionName = lstrModifiedSectionName.Replace(" ", "")
'
' Look for a control that matches the name of this display item and then name of propertydef
'
With SectionControls.GetEnumerator
Do While .MoveNext
Dim lstrKey As String = CType(.Key, String)
If lstrModifiedSectionName.IndexOf(lstrKey) > -1 Then
lobjInterfaceType = CType(.Value, Type)
End If
Loop
End With
It takes the section name (internal name is before any localization), replaces any '-' / ' ' with blank, then looks for any overridden sections containing this string.