When you are responsible for uploading spare parts, you must respect the powerful import tool you are working with. This is true whether you upload spares on a daily basis, or just when adding new parts or vendors, for example.
Before you read any further, STOP! Export all of your current spares before proceeding and then do so with caution. You do not want to be the person who single handedly wiped all the spare parts for your company off the face of the earth.
It is extremely important to be aware that the Table Mapping Fields need to be 'mapped' to corresponding column headers from your import file. (Like when you are asked to pick similar objects from different lists.)
A good rule of thumb is to choose 'Null' if the Table Mapping Field is looking for a numeric response and choose 'Empty String' if the response is a word or alphanumeric.
As a general best practice, never presume that any part of a spares load will ignore anything. Expect that everything can be overwritten and lost. Always prepare for a spares load by first exporting all current spares. Retain that file. Then edit that file as appropriate, while retaining the original. Load your edited file.