The flexible body to be substructured is assumed to behave in a linear elastic manner, as follows
Only linear materials are allowed.
Nonlinear elements within the body (such as gasket or contact elements) are treated as linear and in their initial state.
The body may consist only of 3D structural elements. (You can use 2D elements with care provided that you follow the guidelines given later, particularly with respect to the number of degrees of freedom at the master degrees of freedom.)
Element formulations using Lagrange multipliers are not allowed.
Density or mass of some form must be present in the body.
The body may undergo large rotations, but the strains and relative rotations within the body are presumed to be small.