This is for the information of others who are using InfoPath 2007 in creating forms. Though this has been discussed in this link:
I would still like to discuss on how to do it in 2007 version.
In this form, when split has been selected, Open Point field is required. Click on the required field text box properties, and click on Data Validation. Then add a condition on the data validation.
In the condition, enter two rules such as:When “Split” is selected in the option box and the required field is blank. Like in this case, SWERTodo is the name of the optional field and Open Point is the conditionally required field . Do the same thing with other conditional required fields if there are any.