While API is great for linking Deputy with other apps, you can also use DeXML scripts with Deputy to extend its functionality. More over you can write custom reports as well as custom HTML5 apps on Deputy's framework.
Please note: most functionalities of Deputy will only work with Deputy Paid plans and Deputy Enterprise customers. Whilst a customer is on a trial of Deputy (30-day trial), only the API will work. For customers whose trial accounts have expired, all customization functionalities of Deputy will be limited.