The data tab is use for defining the FetchXML, which will define what sort of records you want to show on your scheduler. You have the option of working with the primary entity or any other entity that the one has a relationship with.
Load Default: Will load the default FetchXML that set to the selected CRM entity. Build: Will let you build the FetchXML, and you will be able to select what fields that you want to display, filter the data, group the data.
Please refer to FetchXML Builder How-to for more information about the Fetch XML builder.
An order for scheduler widget to work as intended it needs to have a two date fields, one numeric field, one bool and two string fields selected in your FetchXML