[A} Fetching one custom field from one document to another can be done while creating the custom field in the Settings module.
[B] While creating the custom field, there is an option "Fetch From" which gives us the option to fetch details of the custom field from the previously created document.
[C] Ex. if you want to fetch the custom field added in OC to be reflected in invoice with there data then while creating custom field of invoice select the data type same as select in custom field of OC.
[D] After the selection on the data type click on drop-down of fetch from option there you can see the the custom field of OC select it and save the custom field
[C} Now if you create the new document, details will be automatically fetched from the other document that you have specified for this particular field.