A very common customization is adding of custom fields. You can add Custom Fields in any Master or Transaction in ERPNext. To add a Custom Field, go to:
and save the Custom Field. When you open a new / existing form of the type you selected in step 1, you will see it with the Custom Fields.
#### Naming
Many times you want your fields to be carried over from one form to another. For example, you may have added a Custom Field in Quotation that you want to include in Sales Order when a Sales Order is created from the Quotation. This is simple in ERPNext, just make sure the fields have the same “fieldname”