This parameter is used to identify the tracefile easily from the directory. When you specify or set this parameter with a value that value becomes the part of the trace file name. So that easily can be identified from the directory. This is a session modifiable parameter and each time when you modify the parameter value the name of the very immediate trace file will get effect of the same.
After putting/accessing a DVD in the drive, while opening the putty.exe it is giving the following error message.
PuTTY Configuration: There is no disk in the drive. Please insert a disk into drive D:.
This parameter is useful when you wanted get the approximate size of the dump file. Suppose you have only limited space available in the disk and if you wanted to check whether your dump will be fit into the available space then you can use this parameter to check or estimate the size of the dump with doing the actual export. Some facts are here about estimate_only parameter.