Among the Candlepower Forums crowd, "16340" generally refers to one particular type of rechargeable CR123-sized cell, but there are other types/chemistries out there, e.g. Li-Co RCR123 cells. "16340" generally refers to IMR-type cells, which are capable of high dischrage rates but have lower capacity than the more typical Li-Co lithium ion rechargeable cells.

I think you'll find this discussion on Candlepower Forums interesting.

I haven't worn out any of my Li-Co RCR123 cells yet, so I have not tried these IMR cells yet. In general, if you want really high currents, then IMR's are the way to go. If not, then Li-Co's still have their place depending on any other priorities you may have.