Using a Data Layer (Google Tag manager)
To ensure maximum flexibility, portability, and ease of implementation, Google Tag Manager functions best when deployed alongside a data layer. A data layer is an object that contains all of the information that you want to pass to Google Tag Manager. Information such as events or variables can be passed to Google Tag Manager via the data layer, and triggers can be set up in Google Tag Manager based on the values of variables (e.g., fire a remarketing tag when purchase_total > $100) or based on the specific events. Variable values can also be passed through to other tags (e.g., pass purchase_total into the value field of a tag).
Rather than referencing variables, transaction information, page categories, and other important signals scattered throughout your page, Google Tag Manager is designed to easily reference information that you put in this data layer. However, explicitly declaring the data layer is optional and if you choose not to implement the data layer, you can still access values from the page using the Variables feature, but you cannot use events without the data layer. Implementing the data layer with variables and associated values, as opposed to waiting for those variables to load throughout the page, ensures that they will be available as soon as you need them to fire tags.
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