Which forum post are you referring to and what restriction do you mean. The doc currently states the following:

At the moment there are several preconditions an indexed field has to meet in order to apply faceting on it. The indexed property must be of type String, Date or a subtype of Number and null values should be avoided. Furthermore the property has to be indexed with Analyze.NO and in case of a numeric property @NumericField needs to be specified.

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