The “Syntax” preference pane allows you to configure aspects of the MultiMarkdown/Markdown syntax.
“Marker for italics” — should the “*” or “_” character be used when applying italics?
“Marker for bold” — should the “**” or “__” characters be used when applying bold?
You can configure whether tabs are equivalent to 4 or 8 spaces when converting tabs to spaces and vice-versa with the “Replace Tabs with Spaces” or “Replace Spaces with Tabs” menu commands.