For desktops:

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K is unlocked CPU (For example: Core i7–9700K)

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X is the Extreme series of CPU (For example: Core i9–9960X)

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XE is the highest core count variant of X CPU (For example: Core i9–10980XE)

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R (Broadwell only) is CPU that fits in BGA sockets

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C (Broadwell only) is the K series but for Broadwell

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T is low power desktop CPU (For example: Core i3–9300T)

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F is CPU without integrated graphics (For example: Core i5–9400F)

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For laptops:

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U is low power laptop CPU (For example: Core i3–10110U)

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Y is ultra low power CPU (For example: Core i7–10510Y)

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H is high power CPU (For example: Core i7–9750H)

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G (Gen 8) is CPU with AMD Radeon Vega iGPU (For example: Core i7–8809G)

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G (Gen 10) is CPU with Iris Pro iGPU (For example: Core i5–1065G7)

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HQ (before Gen 8) is H CPU with 4 cores (For example: Core i7–7920HQ)