Definition required: mixed-level simulation
I was going through the manual of ngspice and came across the following sentence:


  Historically, circuit simulators have supported either an analog simulation algorithm or a digital simulation algorithm. Ngspice inherits the XSPICE framework and supports both analog and digital algorithms and is a "mixed-mode" simulator.


Therefore, this implies that 'mixed-level' simulations take into account both analog and digital signals. Again, from the ngspice manual (Section 1.1.3):


  Modern circuits often contain a mix of analog and digital circuits. To simulate such circuits efficiently and accurately a mix of analog and digital simulation techniques is required. When analog simulation algorithms are combined with digital simulation algorithms, the result is termed “mixed-mode simulation”.