Main Help → All Commands → MPM Grid and Material Points → TartanBorder
The commands creates a border around an object when using a Tartan grid:
TartanBorder xmin,xmax,ymin,ymax
where xmin, xmax, ymin, and ymax give number of extra elements beyond the edges, which may be small or large elements depending on tartan grid settings (add zmin and zmax for 3D). Added elements will match the size of the last cell within the object in that direction. If no Border command is used, a border of 1 cell on each edge is created.
Full details on creating Tartan grids are on the OSUPdocs wiki.