The full screen editor (qed) is a program which allows you to type in text, edit it and save it away in a file. The editor itself treats your text as a series of lines consisting of from 0 to 512 characters. It was designed as a program development editor first and a word processor second.
When operating on text from a file, the editor reads a copy of your file into memory (we will call this a buffer). Any changes you make to your file copy (the buffer) will not affect the original file until you issue a write command to save your buffer. Should you change your mind about saving the particular changes you have made, you may exit without saving, or you may reread a copy of the original file.
The maximum number of lines you can edit will depend on how long each line is and the amount of available memory space. The maximum number of lines in a file that may be edited is on the order of 100 000. Larger files must be split and if necessary rejoined after editing.