Your initial credit line is set the first time you submit invoices for advance. It is determined based on a number of factors, included but not limited to your credit history and your customer profile. It's ok if you have less than perfect credit or if you have been refused a bank loan.
You are free to use the balance remaining on your credit line as long as none of your outstanding invoices are beyond their due date. You can turn your credit line as often as you wish as long as your account is in good standing.