Endurance – The working life of an SSD before its flash cells wear out. The wear out rate is related directly to the number of write cycles a cell has to endure. The smaller a flash cell is physically then the fewer electrons it contains to indicate its charge and hence bit value. The more bits per cell then the same relationship exists. That means that as flash cells become smaller and/or their bit count increases, their working life shortens. 

Here is an indicative table of representative raw endurance numbers for flash with different cell levels and different process sizes:

These are pre-3D NAND flash technology numbers. An SSD’s endurance can be extended by over-provisioning; having spare cells ready to be used to replace existing cells as they wear out.

Low-endurance flash can be used for read-centric applications such as archiving, where data, once written, is not changed.