The use of layers is one of its main features. You use them to modular design the different areas of the outline, so that you will not need to modify the entire outline when you make a change.
Dia is also noted for its ability to anchor objects. With this function you can move an object like a box or an ellipse without the arrow pointing to them being affected by the movement, that is, you will not have to redo the entire outline.
The ways you can draw with Dia range from the usual ellipse, rectangle, the arrow, the polygon, etc..
Dia requires GTK+ Runtime Environment. First install GTK+ and then the application.