The format utility learns of the location of your data file by the following methods.
If a filename is given with the -x command line option, that file is always used as the data file.
If the -x option is not specified, then format looks in the current directory for a file named format.dat. If the file exists, it is used as the data file.
If neither of these methods yields a data file, format uses /etc/format.dat as the data file. This file is shipped with the Solaris operating environment and should always be present.