Absorption Coefficient is related to the medium participating in radiation and refers to the amount of radiation being absorbed over certain length (second term of the RTE equation on the LHS). Absorptivity is surface related phenomenon. If radiation hits an opaque wall a portion is is absorbed and part is reflected (1-emissivity). When a ray travel through a medium it gets absorbed according to the absorption coefficient so the intensity is reduced.
I think what you are referring to it to be provided under Radiation Tab for the walls.
The internal emissivity is used to compute the radiative exchange in the domain whereas external emissivity refers to radiation exchange with the exterior and it is there even if you do not activate any radiation model. -> Internal property.
To answer to the whole questions we need more information about model and if you have transparent walls etc but perhaps the explanation provided is already enough.
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