TL;DR: You should note that the rate of the allowance is set each year by decree, within the limit of 10%.
Rate of the allowance
The rate of the allowance, which reduces the tax base, is set each year by decree. It cannot exceed 10%.
Buildings concerned
The allowance applies to the property tax base on built properties of buildings belonging to sociétés civiles immobilières whose capital is wholly held by Société Poste Immo, directly or indirectly, when these buildings are rented or made available to La Poste by their owners and are exclusively used for one or more activities mentioned in I and the first two paragraphs of II of article 2 of the law of 2 July 1990 relating to the organisation of the public postal service and to France Télécom.