HTML provides the interface for the desktop while HDML provides the smaller screens for the faster access phones. User actions on the cell phone trigger HTTP requests which are answered by sending the HDML code. While most applications will expand easily to include this new data, in some cases new data entry will need to be provided. Text-based data can be displayed and a certain level of interactivity is supported, creating predictable ways for applications within the system to work. This reduces greatly the amount of new information each content provider needs to include, making it similar to the standard ways all ATMs work, no matter where they come from.
Information can be displayed, users can enter data, or they can make a choice from a menu.
Surprisingly, a good bit of information can be displayed and used with so few commands. For example, UPS and FedEx currently offer HDML support for their package tracking. I call up the FedEx application, for example, and enter the tracking number. Much in the same way FedEx's Web interface works, information about my package will be displayed on my phone's data screen. I can also check on flights from United Airlines, and other useful information like restaurant guides, and, in the future hotel and car reservations.