Style vs Template

If you save a score in the personal template folder, you can use that score for a template. Items that are general to the score like instruments and style are applied to score created using the template, specific items like tempo, key signature and any notes or text entered into the score are not used in the new score. If you want different instruments with the same style, then use style.

The way I'd put it is this:

A style is just the group of settings you make in Layout / Page Settings, Style / General, and Style / Text. You can then load that style into an existing score and only those settings change. You can also set that style to be your default in Edit / Preferences / Score and it will be applied to all new scores when not using a template that contains its own style settings.

A template is something very different. It is an actual score - not just the style settings, but the set of instruments you have and any customizations that were done in Staff Properties etc. In fact it really contains the notes and everything, those just aren't actually used when creating a new score from that template. A template cannot be applied to an existing score; it is only something that can be used for creating new scores. When you create a new score from that template, you get basically an exact copy of that score with the actual measures removed and replaced with empty ones.