All right, it's not a customization; it's your own personalization. But the question about the source of data is still relevant. In fact, understanding where the fields come from can answer your question.
You've apparently added these three fields:
Notice that they come from the same table (Inventory module parameters). Therefore it's a single field; the data you see just come from different records of Inventory module parameters table. The field is called Unit; whether it's a purchase, sales or inventory unit depends on other data (namely InventTableModule.ModuleType field).
In your case, you should use a customization to create form controls mapped to these fields and change field label to labels you want in the context where you're using these three instances of InventTableModule.UnitId field.