What is the difference between the IsNotNull and the Switch unit?

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The Switch and the IsNotNull are both operation unit, which means that they have to be placed outside pages.

The Switch unit is used to make an operation chain following different paths depending on the value of its input parameter (just as a SWITCH-CASE construct in programming languages). Usually, one parameter is provided to the unit and depending on the value of this parameter the unit follows one of the different OK links defined for the unit. In order to create the different cases you have to add the "case values". To add a case value you have to use the "case" property in the Properties View and add as many values as the number of the cases you want to manage. After that you can add the OK links to the unit, which can have as target different operations or different pages. On each OK link you have to specify in which case it has to be followed. To do this you have to select the OK link and then select a value from the property "Code" in the Properties View.

The IsNotNull unit is used to check whether the value passed to the unit through the paramter is null or not. The unit can follow only two different paths, depending on the result of the unit. The Switch unit allows to compare the input value with different constant values, the IsNotNull unit allows only to check if the input value is null or not. For this reason it's possible to draw only one OK link and one KO link from the IsNotNull unit. The OK link is followed whe the value is NOT null, while the KO link is followed when the value is null.