Which POS uses a compact flash for storing the application?

The Topaz 310 utilizes a compact flash for storing the application, making it the correct answer. Compact flash memory is known for its durability and reliability, which is crucial in point-of-sale systems where consistent performance and data integrity are paramount. The design and architecture of the Topaz 310 allow it to leverage this technology effectively, accommodating various applications needed for retail operations.

In contrast, the other models mentioned, while they may have their specific storage technologies, do not employ compact flash as their storage medium, which distinguishes the Topaz 310 in this regard. The Ruby2 and Ruby Super System, for instance, might utilize different types of storage solutions that cater to their advanced features and capabilities, but compact flash is specifically tied to the Topaz 310.