STED Saturation Factor

The saturation factor (ς) is a factor describing the amount by which the fluorescence is suppressed by the STED beam. The higher this factor, the more suppression of fluorescence away from the optical axis, the more resolution. At values below 1 hardly any resolution is gained.
The saturation factor depends on factors like the laser power, gating timing in gSTED, and importantly the laser/dye combination. Typical values are in the range of 5 to 50.
For more details see this publication from the group of Prof. Stefan Hell.

More information on the different STED parameters in Huygens can be found on the STED Deconvolution page.