2x4 refers to the USB input and output capabilities of ProFXv3 mixers, with 2 sends and 4 returns.
The ProFXv3 series can send the main mix through 2 tracks out via USB. In other words, the USB output offers a digital copy of your stereo main mix but no individual multitracking.
You can return 4 tracks (aka 2 stereo mixes) to the mixer from your computer. USB 1-2 can blend with your analog inputs for monitoring while USB 3-4 can be assigned to a channel for house music or mixing with the band!
If multitracking is an essential feature for your workflow, you'd be better served by an Onyx mixer.
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