I have a general question about MPCORB data, specifically about the "absolute magnitude" number in MPCORB.dat records.

In MPCORB.dat, the field labeled "H" follows the "Designation" field in the data record. In the MPC description (at http://cfa-www.harvard.edu/iau/info/MPOrbitFormat.html) it's called "Absolute magnitude." In the .dat file, the values of H range from something like 3.34 down to 23 or more.

I want to estimate the apparent magnitude of objects as they come near Earth. Does someone know what the reference distance is for the absolute magnitude?

For stellar astronomy, absolute magnitudes are referenced to a distance of 10 parsecs, or 32.6 light-years. This clearly is not the reference distance for MPCORB objects -- if it were, most would outshine the Moon when in the vicinity of the Earth, say at 1 AU. So it's got to be something different other than 10 parsecs.