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Templates are fragments of HTML that are not rendered immediately when a page is loaded, but can be rendered later. This is useful for speeding up loading times because templates don’t need to be rendered when the app starts. For example, if your app contains a page in a template and the user never visits that page, the page will never be rendered. If you hadn’t used a template, the page would have been rendered even though the user never visited it.
https://developer.mozilla.org/en-US/docs/Web/HTML/Element/template
In Onsen UI, there are two ways of defining pages that the navigator can load. One is by using the template tags e.g.:
<template>
<ons-page>...</ons-page>
</template>
The other way is to define a page in a separate file (without the template tags).
A template isn’t used instead of ons-page. ons-page defines an Onsen UI page; template lets the browser know that it shouldn’t be loaded at startup.
I hope that helps. If you still have questions please let us know.