Daniel Passos nope, it's just a normal string on the server: https://github.com/aerogear/aerogear-unifiedpush-server/blob/master/model/api/src/main/java/org/jboss/aerogear/unifiedpush/api/Category.java#L32

no extra validation is applied to it:
https://github.com/aerogear/aerogear-unifiedpush-server/blob/master/model/jpa/src/main/resources/org/jboss/aerogear/unifiedpush/api/Category.hbm.xml#L12-L14