Top of a given thread. Equivalently, the Table of Contents. There are several icons in each pages. It takes you to the Lobby - the Travel Forum fronmtpage. MAIN button takes you to the main page of a given forum.
POST button is found on the MAIN page of each forum.
You can reply to a given message by clicking on this icon below the message. This is how you contribute to an ongoing dialogue. Takes you back to the table of contents. For example, you can use the forum links drop-down box at the bottom of each page to jump from one forum to another. There are couple of cool icons that you embed directly into your message.