What to select in 'Person Number' field while uploading Customer data using Spreadsheet

I am trying to update Customer Contact data using Spreadsheets. Under the Contacts sheet there is a column called 'Person Number'.

I am not sure what to put here as it's not mandatory that the contact will be an actual person in the System.

Also, is there any Flag in the spreadsheet that we can use to specify this is a 'Customer Update' and not creation of new records

Thanks

Bhavya