Under Drawing Site Line Details in the help you can scroll down a little bit and see a description of how to edit a fence.
You can see that you have control over a fence's rails, boards, posts, post caps, pads and you can also used 3d symbols as fence panels
To Draw a preconfigured fence go to Site Mode, Select Draw->Site Work->Fence
There are many options to choose from.
Drawing a preconfigured fence and editing it will give you a good idea of what the different fence controls do should you wish to define your own.
To define your own for reuse, go to System Options->system library. Pick Fence Definitions as the library type and use "one step" to create a new fence type.
If you have already modified an existing fence on your drawing and you wish to reuse it, you can edit it and use the Add Definition Option which will place the fence in a library for you.