From the screenshot that you had attached, it seems that you wish to constrain geometry to reference planes. The trick is to place a reference plane in the position that you want the geometry to be. Reference planes are the "bones" of Revit families. They control the dimensions and performance of geometry in the family
Some family best practices -
1) Start with reference planes. If you want to create a box 2'x2'x2', then in the plan view you will create reference planes for the length and width, and in a front or side view you will create reference planes at the top and bottom of the geometry
2) Dimension the reference planes. Associate each dimension with a parameter - for instance, length, width, etc
3) If there is something that want do NOT want to allow to change, then select the dimension and click the little padlock to lock the dimension.
4) THEN create the geometry Use the Align command and the little padlocks that come up after the Align command has been executed.
5) Test. Change the values of the parameters. As the reference planes move, the geometry will flex along with them
Can you show me exactly which constraint it is that you wish to edit in the screenshot? I will then be able to give you a more specific solution
Rina Sahay
Autodesk Expert Elite
Revit Architecture Certified Professional
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