There are four types of NAND flash, with an erratic naming convention: SLC (single-level cell), MLC (multi-level cell), TLC (triple-level cell), and QLC (quad-level cell). SLC NAND is used for cache in SSDs and HDDs, but DRAM is more common. In terms of the number of bits of data each scheme's cell can handle, SLC, MLC, TLC, and QLC accommodate 1, 2, 3, and 4, respectively. Software is required to do this, but any variation in the voltages cells can accept plays havoc.
The industry has mostly switched from 2-D to 3-D NAND, though SLC will probably never be made in 3-D. Each "LC" generation is worse with respect to endurance and performance -- MLC is ten times worse than SLC and QLC is one thousand times worse than SLC -- but the yield is greater from the silicon, i.e. profits are higher. TLC introduced a startling drop in write speed as compared to MLC, with QLC being even worse. This is true for both SSDs and USB flash drives. Businesses using enterprise SSDs are the ones pushing for more bits per cell, especially ones which deal in WORM (write once, read many) storage, which is very different than that of PCs.
Some users might be tempted to eschew bargain-basement TLC and QLC SSDs and return to HDDs, but knowledgeable buyers will buy from vendors who supply MLC NAND flash.
Samsung revealed that it is out-of-touch with the SSD industry, but in a good way, asserting in a press release: "Samsung expects that the industry will now focus more on the high performance and reliability of memory storage, rather than immerse itself in a chip scaling race."
Samsung offers 3-D NAND flash in both MLC and TLC, but all of its competitors are concentrating on TLC, with most SSDs only being released with TLC in a race to the bottom. Samsung's press release demonstrated that the company intends to be the leader in the high-end market for both consumer and enterprise SSDs.