When to use it
When you create and/or publish Salsa pages, they will automatically use your default web template. If you have more than one web template, you'll have the option to change the template of a particular page.
Note: Many Salsa accounts will only have one Website Template (because only one Web Template comes with Standard Setup). If you only have one Web Template, you won't be able to change templates.
How to get there
Web templates are managed in the Website Package. Navigate to the Website Package click on the Your Website Template at the top of the page.
Change Your Default Web Template
You'll see your default template under the conveniently obvious header Your default template.
If you only have one Web Template, it will be the default template already - because there's no other option. If you have more than one web template and want to change the default, click Create and view other templates.
Then click a Web Template thumbnail image
And finally, confirm that this is really the default web template you want.
What does it do?
Now, when I create a new Salsa page, the default web template will be applied unless I explicitly change it via the page's publishing tools.