What is the purpose of persistence in regard to custom properties?

If you enable it ("true"), the custom properties you implement with your page tag will also be pushed for all other events on that same page.

For example, suppose you have the following tagging:

var tag = new ATInternet.Tracker.Tag();
tag.page.set({
    name:'Article'
});

tag.setProp("article_ID": "1", true); // Persistent

tag.dispatch();

Here, you would send a page hit named "Article", with the custom property "article_ID" set to "1" (which would therefore define article ID 1).

If you enable persistence as here, and measure clicks afterwards on that same page, the "article_ID" property will also be pushed with the value 1.

When the user loads another page, the persistence will no longer be effective, as it is page-specific. This allows you to cross-reference your custom properties with all other page events from our tools.

If persistence is disabled ("false"), then the "article_ID" property will not be pushed automatically along with the other events on the page.

In case you want to send a list of properties, you can define persistence for all of them:

var properties = {
 "article": {
 "ID": "1",
 "persistent": true
 },
 "theme": "Covid-19"

};

tag.setProps(properties, true);