A tagplan is a document that describes the specification and conventions for tagging your website. For example what to fill in to the chapter fields when do a page view or what to fill in to the recommender field for offers when no algorithm is available.
For all teams that use the NPOTag and are associated with the NPO the tag plan is managed by the data analysis team and available upon request (data-analyse@npo.nl). If it does not exist yet for your platform you can create one together with the data analysis team.
If you use the tag for your own use its still pretty handy to make a tagplan. If you want some input you can always ask.