Well, I’m not really sure they’re used “interchangeably” as such. Either one or the other scale would typically be used. In the sound field, the A-weighting for measuring sound pressure level is often applied to the sound level meter. This provides a measurement that has some resemblance to the 40-phon equal loudness contour, hence the measurements provided will have a closer relationship to loudness than, say, the z-weighting (or zero weighting) measures.
The Reference Equivalent Threshold Sound Pressure Levels (RETSPLs) for sound field testing are provided in ISO 389-7, which are thresholds given under binaural listening conditions and the values it contains are the basis for defining dB HL in these conditions.
Although sounds measured in dB HL are therefore derived using RETSPLs and sounds measured in dB A are measured directly using an SLM, the difference between the two is nonetheless small – on the order of 3dB across most of the audiometric range. This might not be considered a clinically significant amount so it might be acceptable to consider thresholds measured in dB A “as if” they were hearing levels.
References and caveats
ISO 389-7:2005 Acoustics -- Reference zero for the calibration of audiometric equipment -- Part 7: Reference threshold of hearing under free-field and diffuse-field listening conditions