When you use the constraint command there are two choices: Body constraints and Component constraints. Component constraints are very similar to those that you might use in Inventor assemblies and can be applied across components for the purpose of positioning.
Body constraints can be applied to a body and can force geometry on the body to keep a specific geometric relationship.
Try creating a new document and model a simple block. Make sure two side of the block are not parallel.
Next, use a parallel body constraint on these two faces and you will see the body update to keep this new relationship. Even of you use the move tool on either face and rotate, you will see both faces will move.
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