How do I obtain the matrix ordering of the boundaries?

General Discussion
2

I think , you need the list of DoF boundary.

border C1(t=0,2*pi) {x=cos(t); y=sin(t);label=1;}
mesh Th=buildmesh(C1(6));

fespace Vh(Th,P1);
Vh phi, w;
varf vBord(phi,w) = int1d(Th)(w);
Vh on1=vBord(0,Vh);
for(int i=0; i< Vh.ndof;++i)
if (on1[][i]>0) 
cout  << i << " " <<  endl;
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