Custom attributes are attributes, which have nothing to do with the actual filtering or the values, which are outputted.
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Please note, that because DigiZuite™'s own products use the custom attributes "inputType" and "visible", a user cannot add a custom attribute, which has the keys "inputType" or "visible". These two are are a subset of the keys, which have been blacklisted. The full list of blacklisted keys are
'aggregate', 'bindid', 'DefaultToDisabled', 'Distinct', 'editable', 'edlField', 'exifField', 'field', 'fieldName', 'fieldStructure' ,'GroupBy', 'id', 'iptcField', 'isArray', 'itemGuid', 'metaFieldId', 'metaFieldLabelId', 'metaFieldName', 'ObjectReferenceType', 'operator', 'renderType', 'returnType', 'securityType', 'sortDirection', 'standardGuid', 'tooltip', 'type', 'UseAsBigFreetextField', 'valueHandler', 'valueType', and 'visible'.
This list is subject to change in the future.
All of these custom attribute keys are used by the search itself, and therefore, a user users cannot use these keys themselfthemselves.
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.