6/0 is 100 to 140 Denier depending on the manufacturer. However thickness does not give an idea or relative strength of the fly tying threads because materials strength will vary considerably. For Example Semperfli Nano Silk in 6/0 is much stronger than standard Uni Thread.
An 6/0 fly tying thread isthread for flies larger than #14 and for hair work on flies such as Irresistible or making small heads on deceivers. 16 colors. If strength is an issue then look at Semperfli Nano Silk in 100D 6/0, these threads lay flat and are 3 times stronger than any comparable thread and we guarantee it stronger than any comparable thread!