Question

When using the LineItems dropdown search the entire row is replaced instead of the specific cell value that is being edited. Why does it behave this way?

Answer

This behavior is described in the following documentation page: Database Check rule

• Enter value from database If a fuzzy search finds a matching value in the database, this value may be automatically entered into the field if:
  • the values in the field and in the database are different
  • the field is empty.

In addition to this, the dropdown shows an entire row when selecting a value so the GUI actually reflects what is to be done and is self-explanatory:

Usually, there's also a message such as "Quantity not found", hinting at the fact that the specific value does not match one found in the database.
 
The main point of this rule is to make it so that the purchase items on the invoice match those found in the dataset/database. For example, let's say that in the database, there's a record for 3 Anfahrtspauschale being purchased, but the invoice states it was only 1. There is no record found for 3 such items, thus when using the database lookup only the row that was found in the DB is inserted.