CloseFile() will affect a variable that has been assigned to a file with either OpenFileRead() or OpenFileWrite(), making it so that that variable is no longer associated with that file. It is used like so:
dim file file = OpenFileRead("file.txt") 'do stuff CloseFile(file)
That variable can then be used for anything else, like a normal variable, or can be assigned to another file for reading/writing.
When a Basic4GL program closes, all variables assigned to files are automatically cleared. However, it is best to close them using the program.
This page has been recommended for cleanup, integration, etc.