When you register a device on the UPS [1] it returns you a JSON object of Installation with a new id. You can use this id to retrieve an Installation object at any time [2], which contains an array of categories for current device token. We can not guarantee, that device token will be unique in UPS database, that's why you have to use Installation.id for feature purposes if needed.

[1] https://aerogear.org/docs/specs/aerogear-unifiedpush-rest/index.html#246535932

[2] https://aerogear.org/docs/specs/aerogear-unifiedpush-rest/index.html#-1343907292