Only the section of the ISAM file that is actually required is read into the current work file. In F mode, these are the records which are fully or partially displayed on the screen and, in L mode, the line range specified in an EDT statement. On a read operation, the ISAM key is taken over as a line number.
In F mode, every change is immediately transferred to the disk file when the 
key or another data transfer key is pressed. In L mode, the changes are transferred after the execution of an EDT statement. Only the modified records are written back to the file. On a write operation, the ISAM key is formed from the line number. It is not possible to renumber lines when performing real processing.
A file's character set cannot be modified during real processing. The @CODENAME statement is rejected with the message EDT5452.
Line numbers that are longer than the key length cannot be generated during real processing. The corresponding statements are rejected with an error message in the same way as when an excessively large increment is set.
Errors in the file (e.g. non-numerical keys, duplicate keys, illegal byte sequences etc.) may sometimes not be detected until the corresponding records are to be read. Not all the records in the file are read in when the file is opened. Any errors that are subsequently detected are then reported by the command which was executing when they occurred. In such cases, a file that has been opened for real processing is automatically closed.