These functions are reporting-specific functions that deal with the state of a report being previewed.
For example, the notation {Orders.Order Date}
refers to the value of the field in the current record where PreviousValue ({Orders.Order Date})
refers to the value in the immediately preceding record. NextValue ({Orders.Order Date})
refers to the value in the next record. IsNull ({Orders.Order Date})
checks if the field's value is null.
Other examples are PageNumber and TotalPageCount. These can be used to access pagination information about your report.