inputField="this" is most commonly used as a parameter while defining some custom condition for visibility/validity/editability on a field or may be in action for a trigger. With inputField="this", you can fetch the class in which field is defined. For instance, if you would like to add a custom condition on Supplier field under ReqLineItem class, then you can access Requisition Line Item to extract further details in your custom java code.
As Supplier field is defined under ReqLineItem class, so inputField as this will return BaseId for ReqLineItem to you custom java code from you can extract further requisition line item details.
<parameter name="ReqLineItem" inputField="this"/>
Not only "this", you may also pass on any other field value defined under ReqLineItem. For example you can pass on ReqLineItem.Accountings.SplitAccoutings object to your custom code as below:
<parameter inputField="Accountings.SplitAccountings" name="Target"/>
ReqLineItem is just an example, you may use this functionality for any class field. Let me know if you need any further clarification.