Max Modes to Find: You need to specify the number of buckling load factors and corresponding buckling mode shapes of interest. Typically only the first (lowest) buckling load factor is of interest.

Keep Pre-Stress Load-Pattern: This property is available when the upstream structural analysis is nonlinear. This property enables you to keep the loads defined in Static Structural analysis and use them in Eigenvalue Buckling analysis to generate perturbation loads. The default setting is Yes. Setting the property to No requires you to define a new loading pattern for the Eigenvalue Buckling analysis.

Note that at least one load is required in the buckling analysis when the prestressed analysis is nonlinear.