First you create a custom fitting or component and transform it to an iPart factory. Then use the Tube & Pipe Authoring command to prepare the part for publishing to the Content Center. You can also author and publish a normal part (not an iPart) as a fitting.
You can author and publish tube and pipe iParts as either pipes or fittings, but can only author and publish normal parts as fittings.
When you author tube and pipe iParts, you specify:
When you author normal parts, you specify all the previous items, except for the Parameter and iPart table mapping. In its place, you specify a fixed nominal size.