fig.Poner medida; conceder con limitaciones.
http://es.thefreedictionary.com/tasada - citing Diccionario Enciclopédico Vox 1. © 2009 Larousse Editorial, S.L.

It doesn't just mean the WHO constitution provides a list of 'materias', but that those appearing in the list are a sub-set of all 'materias' that might be thought relevant. IOW, the list in the constitution is 'taxativa' (as opposed to 'indicativa'), meaning that only those 'materias' in the list can be considered.

aunque reducida a determinadas materias *tasadas* en la misma Constitución
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although restricted to topics(?) specifically/expressly allowed in the Constitution itself.