When using Theia, understanding the purpose and usage of the primary and secondary navigation panes is a must.

Simply put, these are two vertical navigation panes (fields, or areas; whatever you want to say) which you can use to write straight up HTML or special Theia code. Both of them can be enabled or disabled, so you can use neither or both of them at the same time if you desire.

On a normal installation, only the primary navigation pane will be enabled, and it should already contain the necessary codes to load the category list and the section list. The primary navigation pane is defined as the navigation pane to the left of your content, whereas the secondary navigation pane is described as the pane to the right of your content.

Here is a picture showing the primary navigation pane being highlighted, along with the tool (admin tool 109) to edit it:

And here is a picture showing the secondary navigation pane being highlighted, along with the tool (admin tool 110) to edit it:

And finally, here is all of the special code you can use in these panes to easily display respective modules!