Overlapping objects are stacked in the order in which they are created or imported. You can use the Arrange submenu to change the stacking order of objects.
Arranging objects is not the same as using the Layers panel. A separate stack of objects exists within each named layer, and the Object > Arrange menu commands control stacking only within each named layer. If you haven’t created any named layers, your document contains just one stack of objects on the single default layer. Objects on masters exist at the back of each named layer.
To move a selected object to the front or back of a stack, choose either Object > Arrange > Bring to Front or Object > Arrange > Send to Back.
To move a selected object forward or backward past the next object in a stack, choose either Object > Arrange > Bring Forward or Object > Arrange > Send Backward.