Whilst I do not have a definition for manual exposure, I did find how the 400 Series calculates auto-exposure.
"When Autoexposure is turned on, it will average the intensity of all the pixels inside of a predefined Region-Of-Interest (ROI) and will try to maintain this value at a predefined Setpoint. Both the ROI and the Setpoint can be set in software. In the RealSense Viewer the setpoint can be found under the Advanced Controls / AE Control".
"The ROI can also be set in the RealSense Viewer, but will only appear after streaming has been turn on. (Ensure upper right switch is on). You will then be able to drag a ROI on the image. Corresponding software controls calls exist in the RealSense SDK 2.0 of course".
This information came from a document I found about tuning the 400 Series for best performance.
^^ The above thread was just posted that contains what appears to be a reasonable answer that aligns with my empirical findings, however, I would appreciate official confirmation. This means that exposure = shutter speed, which is arguably atypical.
Quoting the above document, 2.d:
"The exposure units are microseconds, so 33000 is 33ms."