The "u" object is the object used in tag templates to store tag specific config values. You really shouldn't need to access the object. Through mappings, you should be able to overwrite any of the configs that are stored in the "u" object.
If you are not able to overwrite a config value via mapping, I would make sure to reach out to your Account Manager with the name of the tag and the config you're having trouble with so we can make sure to get our template updated.
If you must access this, you can do so via extension scoped directly to the tag you are trying to alter. The scope of the extension is critical because it has to be assigned to the specific tag.
The u object is private and should not be accessed. It's used as a shorthand inside the send function of each tag to refer to that tag's configuration data, such as mappings. The b object is the only one you should ever need to access.
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