Markup: templates/output/04-elements/forms/button/button-service.html
<!-- START BUTTON SERVICE-->
<button class="btn btn--service" title="This is a default title text">
<svg viewBox="0 0 36.717 36.96" role="img" aria-hidden="true" focusable="false">
<use xlink:href="Images/svg/sprite.symbol.svg#icon-write"></use>
</svg><span>Some service button text</span>
</button>
<!-- END BUTTON SERVICE-->
Source:
assets/sass/04-elements/_elements.forms.scss, line 932