This is a little hard to comprehend without seeing your model, and knowing the relationship between nodes 70 and 500. However -

• A CNODE is nothing more than an association of degrees of freedom.
• When you set things up such that "Node 70, CNODE 500, +Y", you are defining that "Node 70" is restrained to "Node 500" in the "+Y" direction. This means that 70 can lift off of 500.
• When you set things up such that "Node 500, CNODE 70, +Y", you are defining that "Node 500" is restrained to "Node 70" in the "+Y" direction. This means that 500 can lift off of 70.

So, depending on your loading, you would get two completely different behaviors from your system with these two different restraint specifications.