What is ASCII Mode?

In ASCII mode a file may be changed slightly to maintain the meaning of EOL (End Of Line) characters.  UNIX machines use a single line break character at the end of each line, whereas Windows systems use a line break and a carriage return.
It is used when uploading text documents such as HTML, PHP, Perl CGI scripts and plain text files.
Perl CGI scripts are particularly fussy about being uploaded in ASCII mode (the extra carriage return inserted by Windows systems messes them up otherwise).