Is statements in sequence until the first evaluates True, when program control is given to the first statement after the Case. If none of the cases are true control will fall to the first statement following the End Select statement. It is good programming practice to include the Case Else statement to handle any data that does not match another case. It doesn't make any difference from where it is called. Control always transfers back to the line following the one that called the Public Sub.
Data validation is important in programming .
Most of the code in scripts, in general, has to do with validating data entry and enforcing business rules. A program shouldn't allow a user to enter invalid data. Programmers have to anticipate user errors and accomodate them with sections of the script. Jakob Nielsen's Ten Usability Heuristics and the articles clustered around it get into some of the issues of User Interface. Giving over part of the UI for prompts is another good thing to do.