Good question. All page hits (set) means that we use the Google Analytics "set" command to set the custom dimension/metric. The "set" command sets the variable for the current hit and all subsequent hits until the page reloads. For example, if the cdim is triggered on page load, and you have subsequent utag.link calls on the page for link tracking, all the events would receive the same cdim, even if it was not present in the data layer on that specific hit (you can think of it like a cache). The "Pageview" scope means that the cdim/metric will only be sent once, as long as the event is a page view/utag.view event. Example: utag.view({page_name:"homepage", country: "UK"}); // if country is mapped to cdim1 with pageview scope, cdim 1 will receive the value "UK" utag.link({link_name:"home button"}); // here, country won't be sent again. If you switch to "All page hits" scope, however, country would still be present, despite not being available in the data object Does that help to clarify things? The GA documentation on Google's support site explains the set mechanism in a bit more detail.
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