In this example we add three buttons to a relative_container.

The first button is positioned at left-top of the relative_container by set only right and bottom.

   elm_relative_container_relation_right_set(layout, bt1, layout, 0.0);
   elm_relative_container_relation_bottom_set(layout, bt1, layout, 0.0);

The second button is positioned at right-bottom of the relative_container by set only left and top.

   elm_relative_container_relation_left_set(layout, bt2, layout, 1.0);
   elm_relative_container_relation_top_set(layout, bt2, layout, 1.0);

The last one is positioned between first and second button. it changes all of edges.

   elm_relative_container_relation_left_set(layout, bt3, bt1, 1.0);
   elm_relative_container_relation_top_set(layout, bt3, bt1, 1.0);
   elm_relative_container_relation_right_set(layout, bt3, bt2, 0.0);
   elm_relative_container_relation_bottom_set(layout, bt3, bt2, 0.0);

Here you can see the full source:

// gcc -o relative_container_example_01 relative_container_example_01.c `pkg-config --cflags --libs elementary`

#include <Elementary.h>

EAPI_MAIN int
elm_main(int argc, char **argv)
{
   Evas_Object *win, *layout, *bt1, *bt2, *bt3;

   win = elm_win_util_standard_add("elm-relative-container", "Elm.RelativeContainer");
   elm_win_autodel_set(win, EINA_TRUE);
   elm_policy_set(ELM_POLICY_QUIT, ELM_POLICY_QUIT_LAST_WINDOW_CLOSED);
   evas_object_resize(win, 500, 500);
   evas_object_show(win);

   layout = elm_relative_container_add(win);
   evas_object_size_hint_weight_set(layout, EVAS_HINT_EXPAND, EVAS_HINT_EXPAND);
   evas_object_size_hint_align_set(layout, EVAS_HINT_FILL, EVAS_HINT_FILL);
   evas_object_show(layout);
   elm_win_resize_object_add(win, layout);

   bt1 = elm_button_add(layout);
   bt2 = elm_button_add(layout);
   bt3 = elm_button_add(layout);

   elm_object_text_set(bt1, "bt1");
   evas_object_size_hint_min_set(bt1, 100, 100);
   evas_object_size_hint_align_set(bt1, 0.0, 0.0);
   evas_object_show(bt1);
   elm_relative_container_relation_right_set(layout, bt1, layout, 0.0);
   elm_relative_container_relation_bottom_set(layout, bt1, layout, 0.0);

   elm_object_text_set(bt2, "bt2");
   evas_object_size_hint_min_set(bt2, 200, 200);
   evas_object_size_hint_align_set(bt2, 1.0, 1.0);
   evas_object_show(bt2);
   elm_relative_container_relation_left_set(layout, bt2, layout, 1.0);
   elm_relative_container_relation_top_set(layout, bt2, layout, 1.0);

   elm_object_text_set(bt3, "bt3");
   evas_object_size_hint_min_set(bt3, 100, 100);
   evas_object_size_hint_align_set(bt3, 0.0, 1.0);
   evas_object_show(bt3);
   elm_relative_container_relation_left_set(layout, bt3, bt1, 1.0);
   elm_relative_container_relation_top_set(layout, bt3, bt1, 1.0);
   elm_relative_container_relation_right_set(layout, bt3, bt2, 0.0);
   elm_relative_container_relation_bottom_set(layout, bt3, bt2, 0.0);

   elm_run();
   return 0;
}
ELM_MAIN()