The Accuracy of the matrix decomposition is specified in the PARFES (Parallel Sparse Direct Solver) tab; it defines the minimum value of the element of a stiffness matrix which is considered to be different from zero (the value of 10-12 is assumed by default). If a number less than the specified accuracy appears on the main diagonal in the process of the matrix decomposition, such a situation is interpreted as the dimensional instability of the system.
RAM usage (%) — indicates the percentage of RAM used for storing the information in the process of solving a system of equations (40% by default).
Moreover, you can also select the stiffness matrix optimization method.