If you have a template made that you want to appear linked to the reference of a particular document, you need to go to: Settings > Basic Settings > References.
.Access the reference of the particular document and in the "template" section select the created template, which has to be linked to the created document type.
Ej. the template type has to be the same as the reference type, you cannot associate an ordinary invoice template to a vendor invoice reference since in this case when displaying the template field from the vendor invoice reference it would only give the option to choose the created vendor invoice type templates.