An event in "Top-Front-End" context refers to projected edges of object surfaces. The X denoted by djsahand is actually short of 2 horizontal parallel bars (solid lines), that has been limited by editing tool of SDN's message box. When combined together, it should resemble a standing hour glass viewing from the top.
For good practices, you should count on either acethedat and/or DAT Achiever, with the latter having complete 3D solutions to further develop your perceptual ability.