Double-acting restraints are those that act in both directions along the line of action. The most commonly used restraints are double-acting.
A CNode is the connecting node. If you do not enter a value in the CNode box, the restrained node is connected using the restraint stiffness to a rigid point in space. If you do enter a value for CNode, the restrained node is connected using the restraint stiffness to the connecting node.
If you specify a gap, it is the amount of free movement along the positive or negative line of action of the restraint before resistance to movement occurs. A gap is a length, so it is always positive.