The min function compares the vector DC1(:,450:Xep) to the scalar DC1(1,j). The result is a vector that is identical to DC1(:,450:Xep) except that all values that are greater than DC1(1,j) are replaced by DC1(1,j). Then this vector is compared to DC(1,j).
If Xep > 451 then DC1(:,450:Xep) is a vector and as I just wrote the result of the min function is a vector. When you compare a scalar to a vector you get a vector of logical values, but to control a loop you should not use a vector of logicals but only one single logical. If you provide a vector, I'm not completely sure, what Matlab does. I would avoid it because you risk that the code does not always do what it should. Perhaps the guy who wrote the code meant something like this:
while DC(1,j) ~= min(DC1(:,450:Xep))
That would make more sense.