The 100 tag is for the author. The computer knows that when it sees the number 100, it should display the contents of the 100 tag as the author of the book to the person looking at the screen. For instance, the 245 subfield a is for the title, but the 245 subfield b is for the subtitle and the 245 subfield c is for a statement of responsibility. In a 260 tag, subfield a is for the place of publication, subfield b for the imprint, and subfield c for the date of publication. Entering the information in the proper field correctly is imperative to understanding the information when you are looking at an electronic record.
Indicators are another component in each field.
Indicators follow each tag number but precede any subfields. Indicators can be used to further clarify the information presented in each field. For instance, in a 100 tag for the author of a book, a first indicator of 0 indicates that the author's name is not in standard inverted order, but is in direct order or is only a first name. The robe a second indicator of 4 in the 245 tag would tell the computer to file the title alphabetically starting with the R in robe, instead of the T in The. More about MARC .