Hello,
the choice depends on the location where your process occurs: e.g., if the emissions happen in an area with a high population density (e.g. a town), then you should choose the flow from the compartment "high population density". If you are not sure, you can select the flow from the "unspecified" compartment. Furthermore, you can check how (and if) the characterization factors change for the same emission from different compartments in the impact assessment method you are going to use.