In Relayter you can add Indesign masterpages to use in your templates. If you have a masterpage with grid-lines, layers, colors, text-styles etc. you can use this in Relayter to generate your Indesign-file with it.

Click on 'Templates' and then on 'Master pages'.

Now click on 'Add master page'.

Give up the name and choose the publication type. And then choose the .Indt masterpage file you want to use. Then click on 'Save'.
​Please note: there can only be 1 page or spread in the Indt. file

Now your masterpage is added to the list and you can use this when creating a template.