Would anyone know how to go about creating a similar design to Sales Orders' and its Items sublist for Custom Records? The idea is I'd have a fully-editable sublist in a Custom Record Subtab, with a mandatory field that, once populated, auto-fills other editable columns. Attached is a snippet from a standard Sales Order form that I'd like to recreate, whether in Suitescript or through GUI methods.

Is using a custom transaction type an option? If not, you can create a child custom record for the sublist level.

Unfortunately I couldn't get an okay for Custom Transaction types, but the child record seems to be an acceptable approach. Thanks for this!