If you're using the Time & Task feature to track and bill your time for your invoices, you can set different hourly rates per task. This rate would be used to override the overall default hourly rate set in your invoice settings.
To set an hourly rate for a particular task:
On the web app, click the gear icon in the top right corner
Click on the task or create your own custom task
Enter the rate in the "Default Hourly Rate" field
Click Save at the bottom