Why there are sRGB/RGB modes in ASTC encoder

In ASTC encoder, we can use -cl to compress RGB texture  and -cs to compress sRGB texture, but why does it need to tell sRGB/RGB texture for astc encoder. From the astc encoder code, I can see some different in processing, but I don't understant why it is needed 

(1) astc encoder tries to fit the texels to a line, it relates to the data distributition, fitting sRGB texure or RGB texture will be not different.

(2) ASTC spec CEM, there is only LDR/HDR mode, no LDR_sRGB mode , so sRGB/RGB will share the LDR CEM.

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