Maybe read the manual? Here's a quote from it relevant to your question:
Line Outputs. These balanced ΒΌ-inch TRS line outputs can be used to route audio to external devices, such as headphone amps, signal processors, and additional monitors. STUDIO1824c: The first
two outputs share their playback streams with both the main outputs and Headphone 1. Outputs 3 and 4 share their playback streams with Headphone 2 for both the STUDIO1824c and the STUDIO1810c. All other outputs have their own independent playback streams.
