A group of fields used as keys for a frequency cap. There can be no more than one frequency cap with the same key.
| Fields | |
|---|---|
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                   The level on which the cap is to be applied (for example, ad group ad, ad group). The cap is applied to all the entities of this level.  | 
              
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                   The type of event that the cap applies to (for example, impression).  | 
              
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                   Unit of time the cap is defined at (for example, day, week).  | 
              
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 Number of time units the cap lasts.  |