Hi alicedgar,
Some advanced applications might require more resources than simple ones. Due to that, we implemented the feature to allow you to choose from a few different tiers that you could use during the load test. We also allow you to start a free benchmark test to determine which machine would be the best for your application. You can find more about this in our documentation:
https://support.smartbear.com/loadninja/docs/scenarios/load-configuration.html#configure
In case you face any issues, feel free to contact us by creating a support case, and providing us with more details.
Greetings,
Tom.