That's ultimately dictated by the FieldEditor user control you reference. Here's an example for a custom SPField I recently worked on:
<wssuc:InputFormSection Title="reCAPTCHA Column Settings" Description="Specify your public and private reCAPTCHA keys along with selecting a theme and default language. For more information on reCAPTCHA and to obtain your own keys go to http://www.recaptcha.com" runat="server">
<Template_InputFormControls>
<wssuc:InputFormControl LabelText="Public Key" runat="server">
<Template_Control>
<wssawc:InputFormTextBox Title="Public Key" class="ms-input" ID="rcPublicKey" Columns="40" Runat="server" />
</Template_Control>
</wssuc:InputFormControl>
<wssuc:InputFormControl LabelText="Private Key" runat="server">
<Template_Control>
<wssawc:InputFormTextBox Title="Private Key" class="ms-input" ID="rcPrivateKey" Columns="40" Runat="server" />
</Template_Control>
</wssuc:InputFormControl>
<wssuc:InputFormControl LabelText="Theme" runat="server">
<Template_Control>
<asp:DropDownList runat="server" ID="rcTheme">
</asp:DropDownList>
</Template_Control>
</wssuc:InputFormControl>
<wssuc:InputFormControl LabelText="Language" runat="server">
<Template_Control>
<asp:DropDownList runat="server" ID="rcLanguage">
</asp:DropDownList>
</Template_Control>
</wssuc:InputFormControl>
</Template_InputFormControls>
</wssuc:InputFormSection>
If I wanted another section to show up I would just add another InputFormSection and then add whatever controls I wanted within that section. The use of the InputForm... controls handles all the layout and formatting for you.