coordinate files

The coordinate list is formed by two files with the same name, but different filename extensions. The first data file contains the coordinates (the filename extension .crd), and the second file, so called the index file, contains information on points sorting (filename extension .crx). By failure of loss of the data file, the coordinates of all the points are lost, but the lost index file can be recovered using the data file at any time. That's why it is necessary to carefully backup the data files, but to backup the index files is useless.