In brief
The atrium, baptistery and palace explain functions beyond the main hall.
Useful to know
The Bishop's Complex in Porec includes a basilica, atrium, baptistery and an episcopal palace. UNESCO notes the combination of classical and Byzantine elements in the basilica. [1]
Instead of just looking at the apse or mosaics, trace the sequence of spaces onto a plan. Different parts serve different activities and are not just rooms added for symmetry. The general ensemble gives a more complete picture of religious architecture than a famous interior photograph. When describing, name the specific part, so that the decoration of the whole place is not attributed without distinction.
Sources
For further detail and to check the information: