The DP4 licence file, DP4.SYS or DATAFIT.SYS, includes the name of your organisation and your licence serial number. It also includes the location of the database files and most importantly the name and location of the SYSTEM database.
The DP4 Install program will ask you to supply a copy of this file when DP4 is first installed, and will create a copy of the file with the database file locations filled in in the appropriate location on your machine. However, it does not fill in the location for other files that will be created/used when you run DP4 programs. You set this information up using DFSETUP as described in Running DFSETUP to Configure DP4.
In 32-bit and 64-bit Windows, the DP4 licence file normally resides in the same directory as the DP4 programs. If you change the location of the DP4 licence file, and are using 4.619 or an earlier release of DP4, you must modify the value of the DATAFIT environment variable.
In later releases of DP4 if the DP4 licence file is not stored in the directory containing SYSLIBT.DLL you must specify a value for the dp4home entry in the appropriate section of the DP4 configuration file as described here. It is strongly recommended that you do not do this.