A template is just a normal document which you can create, edit and update using either Microsoft Word or LibreOffice Writer.
This means that templates can be created by developers and non-developers alike.
There are no special "plug-ins" required for those Word Processing applications... creating fields is as simple as typing, eg: <<firstName>>, in to the template.
The fields can range from simple text substitution to more complex syntax to: fill tables with data; add/remove content from the template; perform calculations; or, to identify and replace images.
Please refer to the Template Guide for your product for a full explanation on the Docmosis syntax.