Intel have given motherboard manufacturers the option to spec out budget-oriented X299 boards without top-end features like quad-channel memory. That means you could get cut-price X299 motherboards specifically designed for the lower-spec Core i5 and i7 X-series CPUs.
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The two Kaby Lake-X chips propping up the incoming X-series CPU range aren’t able to access the platform’s quad-channel memory and only support 16 PCIe lanes. A fully specced X299 motherboard would then have a host of inaccessible features when using those new processors.
Intel’s Frank Soqui explained to me at E3 that they've been working with the board manufacturers to give them a way to differentiate their X299 options from each other.