Before the age of online access to indexed, digitized images of records, there was the old Extraction program. Information extracted from microfilmed records by volunteers was compiled in the International Genealogical Index. Researchers could search the IGI by family name, locality, and extraction batch number. Searching by batch number allowed researchers to see all the extracted information from an individual parish.
Search functions in FamilySearch have evolved, and there is no longer an obvious connection between a batch number and a film number as there used to be in the old IGI. For German researchers, however, help is available. The Christa Siebes website “Suche nach Batchnummern” gives a list of German regions, places and time frames, and provides a batch number search that leads you to the Historical Records section of FamilySearch.org. If someone is interested in looking for records in Soest, Westfalen, for instance, this website provides two options to search by batch numbers. For example, a search for batch number C998921 refers the researcher to Historical Records, with 70,951 results.
Knowing where to look and what to click is definitely an advantage when it comes to German batch numbers. This website can help you with your German research.