Ashutosh_5 :
What does SPD means?
What does XMP means?
SPD mean "Serial Presence Detect", it's a little IC on the memory that stores a table, that the motherboard reads, to determine the memory frequency and latency. XMP means "Intel Extreme Memory Profiles" (yes, the initials are wrong, Intel does that), which is an additional set of table values for not standard settings.
XMP is an overclocking technology. Most boards that don't support overclocking, don't support XMP.
You shouldn't need to know this just to buy DDR4-2666. After all, DDR4-2666 has been standardized. But it was standardized long after the original DDR4 specs. So for a while, the only way to get DDR4-2666 was by XMP. And now you, the buyer, are in the weird position in this weird place in history where DDR4-2666 is available in both versions, being programmed either way, depending on the product you purchase.
But, we have reviews.