deling sector descriptions (and data categories in the NEI): 
ag - agricultural emissions primarily of ammonia and VOC (NEI nonpoint)
agfire/ptagfire - agricultural burning emissions (NEI nonpoint): ptagfire uses point locations and day-specific emissions
afdust/afdust_adj - area fugitive dust emissions of PM (NEI nonoint); afdust_adj are the emissions after meteorological and land use adjustments 
airports - emissions at airports (NEI point)
biogenics - emissions from natural sources (NEI nonpoint)
cmv_c1c2 - C1 and C2 commercial marine emissions for US states and offshore areas (NEI nonpoint)
cmv_c3 - C3 commercial marine emissions for US states and offshore areas (NEI nonpoint)
nonpt - nonpoint (county-level) emissions not included in other sectors (NEI nonpoint)
nonroad - mobile source emissions from off-road equipment (NEI nonroad)
onroad - mobile source emissions on roadways (NEI onroad); 
- sometimes subsectors of RPD (VMT), RPV (VPOP), RPP (VPOP-evaporative), and RPH (hoteling hours-combination truck hoteling) are provided; these are specific subcategories based on the type of activity data used to compute the emissions and the specific processes included
- onroad_ca_adj, when it appears separately, are onroad sector emissions in California based on state-provided emissions
ptfire - Point source wild and prescribed fire emissions in the US (NEI events)
ptegu - Point source EGU emissions (NEI point)
ptnonipm - Point source emissions not included in ptegu, pt_oilgas, or airports (NEI point)
pt_oilgas - oil and gas emissions from point sources (NEI point); also includes offshore oil platform emissions
np_oilgas - oil and gas emissions from nonpoint sources (NEI nonpoint)
rail - railroad aka locomotive emissions on railways (NEI nonpoint); note that rail yards are in ptnonipm
rwc - residential wood combustion emissions (NEI nonoint)

Non-US emissions sectors:
othafdust/othafdust_adj - Canada area fugitive dust emissions; othafdust_adj are the emissions after meteorological and land use adjustmentsothar - Non-US area sources
onroad_can - Canada onroad sources
onroad_mex - Mexico onroad sources
othpt - Non-US point sources; as of 2014fb_cdc, does not include any offshore CMV or oil platform emissionsptfire_mxca - Point source wild and prescribed fire emissions in Canada and Mexico
cmv_c1c2 and cmv_c3 also include emisisons from outside of the US