Hi,
The boostrap-responsive.css is one of the available .css files of Twitter Bootstrap package that we do integrated in our T3 system plugin. It is compiled from the boostrap-responsive.less which simply gets the less css from the Bootstrap files (i.e)
// Large desktops
@import "../../../plugins/system/t3/base/bootstrap/less/responsive-1200px-min.less";
// Tablets to regular desktops
@import "../../../plugins/system/t3/base/bootstrap/less/responsive-768px-979px.less";
// Phones to portrait tablets and narrow desktops
@import "../../../plugins/system/t3/base/bootstrap/less/responsive-767px-max.less";
And we use these standard grids of this boostrap file to apply on almost layouts of our template. However, during the template development, we will need to customize some grid styles to make the responsive front-end layout look closer to our template design concept on mobile/tablet or even iPhone view.
So, our developers will define the specific css styles to override the available grid ones (i.e logo, navigation, sidebar, off-canvas menu …) into the templates/ja_template/less/template-responsive.less file. This less css of this file will be compiled to the template-responsive.css file as you mentioned above.