It is my understanding that you are looking for more clarity on what the subplots in the stepplot represent.
The different subplots generated after using 'stepplot' give information about the response of each output with respect to each input. The plots should be looked at individually.
A row of plots signifies the response of a single output against all the inputs. Whereas, a column of plots represent the response of all outputs against a single input.
For more information, please refer to the following example: