What is EXX operator ?
As described in the manual, we have implemented the HSE hybrid functional form as in the reference papers. The default parameter set refers to the HSE06 hybrid functional, i.e., 25% of EXX and screening parameter of 0.11 Bohr-1.
Note that the fraction of EXX and the screening parameter value can not be changed in the GUI, but can be easily modified in the script, see the manual for details. In that sense, you can design your own HSE, e.g., HSE03, or any other that would suite your calculation needs.