  Immersive reality technologies, such as Virtual and Augmented Reality, have
ushered a new era of user-centric systems, in which every aspect of the
coding--delivery--rendering chain is tailored to the interaction of the users.
Understanding the actual interactivity and behaviour of the users is still an
open challenge and a key step to enabling such a user-centric system. Our main
goal is to extend the applicability of existing behavioural methodologies for
studying user navigation in the case of 6 Degree-of-Freedom (DoF).
Specifically, we first compare the navigation in 6-DoF with its 3-DoF
counterpart highlighting the main differences and novelties. Then, we define
new metrics aimed at better modelling behavioural similarities between users in
a 6-DoF system. We validate and test our solutions on real navigation paths of
users interacting with dynamic volumetric media in 6-DoF Virtual Reality
conditions. Our results show that metrics that consider both user position and
viewing direction better perform in detecting user similarity while navigating
in a 6-DoF system. Having easy-to-use but robust metrics that underpin multiple
tools and answer the question ``how do we detect if two users look at the same
content?" open the gate to new solutions for a user-centric system.
