templating confusion

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Hi,

I am looking at your demo forums where you give admin access. i'm a bit confused on the whole templating thing, this is what I understand of it.

1. There are replacements that I can add dynamically to the site and they are stored in the db so I can modify them live.

2. there is a set of default templates for all the sections of the forums application which are also stored in the db.

Question:
If I create my own template, how do I apply the template to a specific page or section, since when I add a template it just adds it to a custom template section and I see know where how I can "overrride" the default settings?