This menu is the most important feature of Graph'O->Mat. It helps you to place your graph in different manners. In this section, only a short explanation on the functionalities of the algorithms is given. You can find detailed information on these optimization algorithms on page in section 3.
The first option named 'Standard' looks for minimal edge-length without moving the vertices. The 'Circle' option places all vertices in a circle and applies the 'Standard' algorithm. 'Centered 2-Circle' is a variety of the circle algorithm and does the same thing with a vertex in the middle and two shifted circles instead of only one. The 'Cross' algorithm takes care of the number on crossings in the graph and tries to decrease them. The 'Spring' algorithm simulates the edges as if they were springs and moves the vertices to a better distribution. Same thing does the 'DynaSpring' algorithm, but as the name already says dynamically and has therefore more efficiency in most cases.
You cannot just say that one of these options gives you the best result. It really depends on the kind of graph you have to place. Best thing is to try the different algorithms and to sometimes repeatedly choose the 'Standard' or the 'Cross' option to get best results.