Space coordinates of points in mesh

i have developed a mesh-model of a 3D tank in PDE Toolbox. For specifying the boundary conditions, I need the space coordinates of the points along the free surface/upper boundary. When using the command meshToPet in the PDE Toolbox, I am able to get the space coordinates of the points in the mesh. If I have 205 points along the upper boundary, do the first 205 rows of the p matrix correspond to these points along the upper boundary? Does the following command call the points along the upper boundary?

x=p(1,1:205);

If not, how do i call the elements along a boundary/edge? In what order are the points specified in the p matrix?