Hi, I was wondering…
What is the purple template flag on SOPs used for?

The purple flag indicates what node will be rendered. It could for example be a hires version that you don't want to display in the viewport, but you want to render it.

OK, thanks.

Actually the purple flag in the template slot means that the template is selectable. This way you can use the templated geometry to make selection and apply operations. By default templates are only for visual reference and for snapping. You get this by Ctrl -clicking on the template flag.

The render flag is also a violet color but is on the display flag. It can be created by Ctrl-clicking on the display flag and it works as mentioned above.