A support is a "boundary condition", a displacement is a "boundary condition". So numerically a support and a displacement are the same thing. A support can be thought of as a displacement of zero magnitude. For example, a "Y" support could be modeled as this displacement vector: (free, 0.0, free, free, free, free).

CAESAR II complains if you try to over-specify a boundary condition (so don't do it). If you need to specify a displacement, remove the support and properly specify the displacement vector(s).

Supports simplfy the input:
- It is much easier to specify a "Y" support than the above example displacement vector.
- You can't specify a "+Y" using a displacement vector.
- You can't specify a "gap" using a displacement vector.

Supports and Displacements each serve their own purpose to simplify modeling. However, they are both boundary conditions that restrict movement in specified degrees of freedom.