What is an ordered file?

What is an ordered file? -- Linux general technology-Linux technology and application information. For details, refer to the following section. To compare two ordered files, run the comm command.
Syntax: comm [-123] file1 file2
Note: This command compares two sorted files. File1 and file2 are sorted files. Comm reads the two files and generates three output columns: only the rows that appear in file1, only the rows that appear in file2, And the rows that exist in both files. If the file name is "-", it indicates reading from the standard input.
Option 1, 2, or 3 suppress the corresponding column display. For example, comm-12 only shows the rows that exist in both files; comm-23 only shows the rows that appear in the first file but not in the second file; comm-123 then nothing is displayed.

What is an ordered file !????