I am working in Odoo v8.

I am creating my own sequences for sales invoice, sales invoice refund, purchase invoice and purchase invoice refund.

Do I need to create a sequence_code for each one? Or just one ("Sales entry") for sales invoice, sales invoice refund and another sequence_code ("Purchase Entry") for purchase invoice and purchase invoice refund?

What is the importance of sequence_code? I see it is related to a journal, so: Is it needed for a corrent accounting?