Confuse with the following, need further advise :
Customise Form for Standard Doctype, and export customisation will allow us to export this form customisation to the Custom App.
We should not modify directly for standard doctype (e.g. : Sales Order or Sales Invoice ), in fact, we should do as point (1) as above mentioned.
For custom doctype, we should modify directly at custom doctype instead of custom form. Correct ?