It has no meaning, it is just a file extension. It is some people's convention to name files with a .inc extension if that file is designed to be included by other PHP files, but it is only convention.
It does have a possible disadvantage which is that servers normally are not configured to parse .inc files as php, so if the file sits in your web root and your server is configured in the default way, a user could view your php source code in the .inc file by visiting the URL directly.
Its only possible advantage is that it is easy to identify which files are used as includes. Although simply giving them a .php extension and placing them in an includes folder has the same effect without the disadvantage mentioned above.
与恶龙缠斗过久,自身亦成为恶龙；凝视深渊过久,深渊将回以凝视…
