This tab is found under the "General" tab, within "Translations" and is only visible to you. It allows you to set Right-to-Left or Left-to-Right settings for the Translation Language.
The Translator cannot see this tab. Left-to-right is the default and will be the setting when this option is un-checked. If you are sending a Translation Language to a Translator for Japanese or any other Eastern language you can check the Right-to-Left option and when they load your project in the "Translation Assistant" they will be using a Right-to-Left editor.
The Html Header field is left for you to add any specific html codes that you might need. For example, if you are using Unicode in your translation, then you might add:-
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
to the header. This will be added to the header of every page. You can also add in scripts or any other supported html codes.