The difference between RPA and RDA is that the RDA is an automation solution that assists the agent in handling simple repetitive tasks. The agent plays a role in facilitating when the automation is triggered and stopped depending on their work flow.
An example of that would be an agent receives a call and needs to retrieve all necessary info related to the customer that is calling. Instead of the agent manually retrieving the data from various applications, the RDA assists in gathering this information in parallel to the agent speaking with customer.
The agent is the one that determines when the data is retrieved and what particular information is gathered by providing an account number to the RDA to assist with.
The RPA is a headless operation in comparison. There are no agents to instruct the robot when to collect the information. There are no users interacting with the robot. The design of the automation is totally self-sustaining.
Suppose you have a massive amount of records that require updating, but the record access is mutually exclusive. Mutual exclusion meaning only one agent can access the record at a time. You wouldn't want to update these records in the middle of a work period, because that would disrupt the call flow of the agents and it would not prove helpful if the account record was already in use by one of the human agents.
Instead, you would create a headless (no user interaction) automation to update the records during off peak hours, i.e. (2 AM in the morning). This headless automation could be started with the use of the Windows Scheduler that starts the Robot at a specific time every night, or every 1st and 2nd Tuesday of the month which ever you prefer. The Robot would start from the task scheduler and perform the necessary updates while the agents are not accessing the records. This headless operation is called an RPA.
Both RDA and RPA are solution automations, but designed with two different task in mind.

​Provides an automation authoring experience designed for rapid development and delivers the most powerful application automation and integration capabilities available today. The visual design surface makes it easy to create workflows and apply business rules through shapes or by recording workflows. 

Intelligence and robotic automation help enterprises maximize the value of their two biggest investments, people and technology, by understanding how users interact with applications. Pega robotic process automation, desktop automation, workforce intelligence, and case management together to enable more productive employees and a better customer experience. Unlike RPA-only solution providers, Pega offers end-to-end Robotics, CRM, and BPM for any digital enterprise,  giving organizations the power to:

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