Need an explanation of setting Boundary Condition in pdepe (PDE Solver)

I am trying to solve Advection Diffusion equation using pdepe (PDE solver) in Matlab. I am not clear why we are writing pl=cl-1.0 in boundary condition. At left boundary, I am setting C(0,t)=1 and C(L,t)=0.1.

I need an explanation of it.

clear
  close all
  %P(1)=1;
  P(2)=1;
  L=0.01;
  maxt=40;
  m=0;
  t=linspace(0,maxt,100);
  x=linspace(0,L,50);
  sol=pdepe(m,@PDEfun,@ICfun,@BCfun,x,t,[],P);
  c=sol;
  figure(1)
  surf(x,t,c);
  xlabel('distance x')
  ylabel('time t')
  zlabel('species c')
  figure(2)
  hold all
  for n=linspace(1,length(t),10)
      plot(x/L,(c(n,:)))

    end
    %legend([t = 1], ['t'=length(t)]);
    xlabel('x/L')
    ylabel('C(x,t)/C0')
    grid on;

    %.........................................
    function [g,f,s]=PDEfun(x,t,c,dcdx,P)
    D=0.000001;
    k=0;%-0.0167;
    U=0;%.001;
    g=1;
    f=D.*dcdx;
    s=-(U*dcdx)+(k*c);
    end
    %..................................
    function c0=ICfun(x,P)
    c0=1.0;
    end
    %................................
    function [p1,q1,pr,qr]=BCfun(x1,c1,xr,cr,t,P)
    %c0=P(2);
    p1=c1-1.0;   q1=0;   pr=cr-0.1;   qr=0;
    end