In the sales process, Service Qualified Lead falls into the category of the hottest opportunities.
An example of service qualified lead is a customer who tells their customer service representative that they'd like to upgrade their product subscription or receive an additional service.
At this point, the customer service representative would up-level this customer to the appropriate sales team or representative, or do the closing on their own, depending on circumstances.
Often this type of leads appears as a part of the upselling process.