An atomic rule item.
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 Rule variable name. It should match the corresponding key name fired by the pixel. A name must begin with US-ascii letters or underscore or UTF8 code that is greater than 127 and consist of US-ascii letters or digits or underscore or UTF8 code that is greater than 127. For websites, there are two built-in variable URL (name = 'url__') and referrer URL (name = 'ref_url__'). This field must be populated when creating a new rule item. | 
| Union field 
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 | An atomic rule item composed of a number operation. | 
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 | An atomic rule item composed of a string operation. | 
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 | An atomic rule item composed of a date operation. |