What is the fine-grained topic of the following text: In response to these competitive forces, we strive to maximize the value of the Mastercard system (including the dollars spent on Mastercard cards, the number and types of cards in circulation, and the number and types of merchants accepting Mastercard cards) by setting default interchange rates at levels that balance the benefits and costs to both cardholders and merchants. Only return the topic.
Maximizing the value of the Mastercard system in response to competitive forces