Endcap is placed at right or left most boundary filler cell for isolation of routing .
Route is not beyond endcap area.it does not allowed to routing come out side beyond endcap. i think the name itself its defintion.
Add endcap.tcl modified script enable to add a double coloum of tie fillercell (dummy poly gate for isolation) & endcap fller cell at the boundaries of each blockage areas define in the top including boundaries of the top itself.