Join files are a special kind of logical file you can use to combine, in a single record, the fields contained in two or more records (in files elsewhere). Join files have the following limitations:
You cannot open a join file for update or output. You can only open a join file for input. In other words, you can only read a join file. A join file can contain only one record format, as opposed to nonjoin logicals, which can contain many record formats.
You can join two records only if the joining fields have the same type length and have equal values. If you define key fields for a join file, all key fields that make up the key must come from the first file listed in the JFILE keyword (see below).