No these arent for storing pornography :-)
This is an older rating system that is based on the read write speed of a CD drive. A CD drive works at 1.2 Mb per second.
A 6x would be 7.2 Mbits/sec
A Speed Class 2 card is roughly equivalent to a 13x card.
This is an older standard and rarely used anymore.
They arent compatible with the UHS-I bus but can be used with the Speed Class cards.
