When trying to delete a custom Sales Order form, an error is returned: This custom form cannot be deleted because it is referred to by other custom forms.
This would happen when the form is set as a default form for Checkout. To verify, perform the following steps:
1. Navigate to Setup > Site Builder > Set Up Web Site.
2. Edit the Web Site.
3. Click the Shopping tab.
4. In the Checkout Preferences section, check the form selected in the Sales Order Type.
5. If this is the form that the user is trying to delete, select a different form.
6. Click Save.
7. Navigate to Customization > Forms > Transaction Forms.
8. Edit the Custom Form.
9. Under More Options, select Delete.