If I needed to visually explain how this custom parameterization works for Azure Data Factory resource, I would picture it this way. Before you solely relied on publishing your ADF code from your collaboration Git branch to the adf_publish branch where ARMTemplateForFactory.jsonand ARMTemplateParametersForFactory.json files live and get further deployed to other environments. You had some flexibility to parameterize your deployment or run some custom code to update ARM templates before they get deployed.
With the introduction of the ADF custom parameterization, you have an additional JSON file arm-template-parameters-definition.json that you can use to define rules to add supplementary parameters to the main ARMTemplateParametersForFactory.json file. There is a very important statement on Microsoft documentation site that explains how this new file operates, “A definition can’t be specific to a resource instance. Any definition applies to all resources of that type”. It’s like a garden rake that will collect all the leaves or none, i.e. if your rule defines a JSON property, let’s say “timeout” of your ForEach loop container, then all timeouts will be scooped into ARM template parameter file.
Read on for the full explanation as well as an example.