IGI Batch Numbers
When parish records were input into the IGI it was done in batch which may correspond to a register or group of registers. By carrying out a search on familysearch with, for example, a surname and a batch number you can find all the christenings or marriages in that batch relating to that surname - and you can refine the search further by date, etc. This is very useful but familysearch provides no tool for determining the correct batch numbers to use for a particular parish or town.
Rootsweb includes a list of batch numbers (which may not be complete)