Our high-performance network music player offers incredible sound combined with effortless hi-res streaming and playback, for an immersive experience and convenient access to a wealth of music
Essentially a volume control so you can connect directly to an amplifier. Different form factor as well which may work better for some. Looks like the connections are the same.
Here is the answer from James Cook, a Product Marketing Manager at dCS:
Both are high-performance network music players designed for hi-res streaming and playback, so they are very similar in terms of performance, features and capabilities. The Lina DAC X offers an alternative form factor for listeners who prefer the full width design and physical volume control. This sits alongside the Lina DAC, for those who prefer a more compact and versatile form factor. Either way, listeners will be able to enjoy the same exceptional clarity and benefits offered with dCS technologies such as the Ring DAC.
The DAC X does not currently have AirPlay support, though we intend to add this in future via a firmware update.