This is optional field. You can enter a literal constant value that appears as the default parameter value when the system presents a parameter prompt form to a user invoking this data transform.
For example, when this data transform is used in an Apply Data Transform action in another data transform, the user can bring up a window in which to specify the input parameter values. That window displays these default values.
In addition to the above reply by Mr.Santanu, I would like to add that:
1. We can use the parameters either as a single valued or as a Page structured. However we need to declare the page name under the pages and classes tab of the data transform.
2. For single valued propeties based on the type i.e., integer, string or boolean values, we need to set the corresponding values.However, there are some points to be made note of with boolean values which are as follows:
The values for Boolean parameters are the same as Boolean properties:
The preferred values are the text constants "true" and "false"
The following are equivalent to "true": any non-zero numeric value (integer or double), the text constants "on", "yes", "t", or any value starting with a "y" in any case
Any other value corresponds to "false"
3. All these default values appear when the data transform is referred in some other activity or data transform etc and we can either use these values itself or can use parameter values of the calling activity.