Lloyd,
You can set the "On Account" limit by account, so some accounts could have a higher spend limit.
Usually this "On Account" would be used for B2B and have a time period associated, e.g, Set a time period of 30 days and a spend limit of $1,000 per period. This would allow the customer to quickly place as many orders as they wish in that time frame that have a total spend of less than their limit, then make a single payment on receipt of a consolidated invoice.
"On Account" payment methods can be used in parallel to traditional CC payments so you can for example process B2C and B2B, by only setting your business/trade customer to have the "On Account" as an eligible payment method. Other customers pay at order completion time.
Chris.