This tool is used to apply a fixed length to a line or a fixed radius to an arc.
To use the tool, identify either a line or an arc. This will bring up the Fixed Value form, with the prompt either Length or Radius like this:
At this point, you can either enter a numeric value or a variable, such as =H, in which case the Variable Properties form will be opened:
Here, you can enter a suitable Prompt and a value or an Expression.
Once the constraint has been added, the line length or arc radius cannot be altered.