Using a Line as a 'Symmetry Inference'
While there is no dedicated 'symmetry line' tool, as symmetry is technically and quantitively defined by Geometrical Dimensioning and Tolerancing (GD&T) principles and symbols, you may however add a line which only represents 'Implied Symmetry' - Examples are shown below.
Three ways to make a representative 'Implied Symmetry' line ( Third way preferred )
1. On the model, In a sketch, create a Layout line by checking the Create Layout Curves option.
A sketch construction line can also be used changing it after exiting sketch to a line, then to a layout line in it's properties. Select and change True > False
2. On a model, with a 3D line, create a Layout Curve line. ( If a line or construction line is used, change its properties as above)
3. On a model face in a drawing sheet view ( as shown ), create a Layout Curve line - this the preferred way due to the option of creating a line in the drawn view only
Now change its Line 'Style' properties
Adjust the length by dragging in a sketch plane
Note: Structure Browser listed in both the Model and Drawing Sheet, the curve line may have a separate independent style in Model and Sheet with renaming for identification.
Another possible problem with this symmetry line is with model changes - it will not automatically relocate if model features / faces are moved requiring probably separate repositioning.
Defining Symmetry using GD&T symbols - 2 ways
1. Annotation GD&T manual creation
2. Encoded GD&T logic
1. Using the built-in symbols / dialogue from Dimension > Annotate GD&T
2. Encode with GD&T semantic logic (defined by ASME Y14.5 GD&T (2009) standard and ASME Y14.41 containing additional requirements for tolerances defined in digital models
Please take a look at the GIF below for a demonstration of the application principle
Note: If a GD&T feature / face is moved, breaking a defined relationship, an error red X is shown next to the relevant symbol in the structure browser
If an incorrect symbol is used i.e a positional instead of a symmetry symbol, an error is also shown