Web service subscriptions entitle the user to a certain number of Map Views. For example, a Planet Basic Web Service subscription entitles the subscriber to 500,000 map views per month. Every time the end-user application loads basemap data (in the form of map views) from Planet servers, Planet counts that against the user's subscription.
A map view consists of 15 map tiles, with a specific map tile consisting 256x256 pixels. A map view is derived from an average size of a browser window on a laptop screen - this is a general, though unofficial, industry measure of map service consumption. Every time the application makes a request for a map tile, whether it has been previously requested or not, this counts as a map tile view. Applications and browsers may have different caching mechanisms that may impact when a map tile is requested - please refer to application specific documentation for details.
Map views (and thusly map tiles) are counted regardless of what resolution or zoom level is being browsed.