▪You have different card types in different files; for example, cards 1 and 2 in the file data1 and card 3 in the file data2. In this case, merging is by serial number and, optionally, card type and trailer card sequence number.

▪You want to merge a field of data from an external file into records from the main data file. For example, you might have a file of manufacturers’ codes which refer to a number of products. If each record in the main data file contains the product the respondent preferred, you might want to merge the appropriate manufacturer’s code from the external file into the main data in the C array. In this case, merging is based on finding matching keys in the main record and the records in the external file.