Custom attributes are attributes, which have nothing to do with the actual filtering of the values, which are outputted.
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Prime examples of custom attributes, which the DigiZuite™ products Media Manager and Video Portal use, are 4 Input Type and 8 Visible filter. Both of these have no impact on the search itself, but the products use these attributes to decide which filters to use and how to collect the input from the user.
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This list is subject to change in the future.All of these custom attribute keys are used by the search itself, and therefore, users cannot use these keys themselves.
Furthermore, a key MUST be unique and MUST start with a letter or underscore. After the first letter a key may have any number of letters, underscores, or numbers.
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A custom attribute can be defined during creation of a filter or value field:
Creation of a filter.
and edited/deleted in the The Details panel.