CurrentInvoiceLine_CreateFromDataArray What is the number parameter?

From the SOAP API human readable documentation for CurrentInvoiceLine_CreateFromDataArray, a parameter 'NUMBER' is specified.

First question: Is 'Number' required or is it not required for the creation of invoice lines?

Second question: If Number is required, what is the data?, Please give an example.

Please give a fullfilling answer to these questions.'

 

Thanks

Nicholai