Drag and drop an App Builder custom dialog object to see an example of attachments. Each of the buttons at the bottom of the custom dialog are attached to the top and sides of their enclosing dialog panel. They are attached five pixels from the top of the panel and varying percentages from the left edge of the panel (Button1 left edge 10%, right edge 30%; Button2 40% and 60%; Button3 70% and 90%).
The left edge of Button1 will always be 10% from the edge of the panel and the right edge of Button1 will always be 30% from the edge of the panel. Button1 will therefore always be as wide as 20% of the total width of the panel. Button2's edges are 40% and 60% from the left edge of the panel; Button3's edges are 70% and 90% from the left edge of the panel.
The three buttons will grow and shrink as the panel grows and shrinks, and the distance between them will always be 10% of the total width of the panel.