Well you focus on the what (HCl/NaOH) means rather than focusing on the question...the question remained unanswered but I will explain to you what buffer capacity(HCl/NaOH) means
No, I am focusing on what you mean - it is impossible to answer the question without understanding it
And your post was - at least for me - far from being clear.
Experimental buffer capacity can be calculated from the slopes of the graphs of pH vs NaOH added or pH vs HCl added
Experimental Buffer capacity= 1/mHCl * ConHCl / Vbuffer
Assuming you started with mentioned plot, slope = ΔpH/ΔV, so buffer capacity is
Experimental Buffer capacity= ΔV/ΔpH * ConHCl
Now, when it comes to units volume cancels out and you are left with concentration/pH - if we assume pH is unitless, what is left is concentration - so you were OK from the begining. Not knowing what you mean by "slope" there was no way of checking.