An INS file is an Internet Communication Settings file. And from using edit to look in the file, all it looks like is a place where information is stored. All it is is information for the Internet Connection Wizard Settings.

And to answer your other question, it shouldn't require multiple INS files. Freewwweb.com, free isp I used to use before it started getting busy signals, created an INS file and all the email and dialup networking settings are in that file.