We can source a field value from a record to a custom field in another record. For that, we can use the ‘Sourcing and filtering’ option in the custom field setup.
Example :
Need a custom item line field in the Purchase order that sources value from the corresponding item record.
Create a custom transaction line field. Apply this field to the required transaction(here Purchase order).
Display type can be set to inline text or disabled as it is a sourced field.
In the ‘Sourcing and filtering’ tab, select ‘Item’ as the ‘Source List’. And select the desired field from the ‘Source from’ field dropdown.
The custom field type and the type of the sourcing field should be the same.