Sometimes data can be lost on commercial disks. The main problem with this method is it overwrites any program currently in memory, so if you were looking for a disk with room, look for it before entering the program. The benefit with this method is that the fastloader prevents the directory from wiping out your program. On some fastloaders and wedges you can use the space bar to suspend the listing of the directory, and pressing it again resumes the listing. The bottom line is the number of unused blocks on the disk.
Files are data on a disk, sometimes they are programs.
All files are recorded in chunks of 254 characters, these are called blocks. A computer picture takes about 40 blocks. They a re tagged deleted so they can be overwritten when another file is added to the d isk. Some programs such as the first one are complete in one file while others like the second one use several files. Usually the first file in the set is the one that loads or 'boots' the program.