I read in the reference  below that
An infinite, non-interacting two-dimensional gas of bosons has no phase transition and never develops spontaneous coherence. However, adding interactions leads to the Berezhinski-Kosterlitz-Thouless(BKT) transition, below which power law correlations, and super fluidity can develop. For a finite, trapped two-dimensional system of non-interacting particles, macroscopic occupation of a single particle state can also occur.
This article is regarding the Condensation of Exciton-polariton which has been considered as 2D Bose Einstein condensate.
This means, if we introduce interaction (trapping potentials) in a finite 2D system, it can undergo a phase transition that is similar to BE condensate. But I thought even a 3D bosonic system requires a trapping potential so that they can occupy a quantized energy levels and reach the critical phase space density to undergo the macroscopic occupation. In this respect, how is 2D condensate different from 3D condensate ? Also, reference  emphasizes similar requirement for BKT in 2D. Can anyone explain what point I am missing here. Any comments are highly appreciated. Thanks