Hi Henry,
You can find descriptions of the boundary conditions and their required user inputs within the User Manual. You can either create a local copy yourself using Python’s sphinx package and the local documentation files contained within the doc directory of PyFR’s root directory, or a current copy of it can be found on PyFR's website at
http://pyfr.org/user_guide.php.
As Frank has already indicated the ac prefix is used to denote the artificial compressibility scheme used in PyFR’s incompressible solver. The others describe boundary conditions for the compressible solver. There are conditions for riemann characteristic invariant, no-slip adiabatic walls, no-slip isothermal walls, slip walls, subsonic inflow in which rho and velocity are prescribed, subsonic inflow in which total pressure and total temperature are prescribed, subsonic outflow in which static pressure is prescribed, supersonic inflow, and supersonic outflow. Hope this helps.
Best Regards,
Zach Davis