Assembly: Microsoft.SharePoint.Taxonomy (in Microsoft.SharePoint.Taxonomy.dll)
The Enterprise Keywords TermSet is always open and cannot contain hierarchical taxonomies. A TaxonomyField object that points to the Enterprise Keywords TermSet allows multiple values by default.
When editing a TaxonomyField object with this property set to true, new Term objects can be added to the Enterprise Keywords TermSet by selecting Create New in the Suggestions drop-down window in the TaxonomyField editor control.
Since the TermSet doesn't support hierarchical taxonomies, the Term Picker dialog box cannot be launched to pick Term objects for a TaxonomyField that has this property set to true.