As described in Constructors for New Windows, modal means that the user must interact with the window immediately. Clicking outside the window produces an error sound. Script execution stops until the user responds to the window.
New Window() with the Modal message. This creates a new window in which you arrange other display boxes.
Column Dialog() constructs a window for column role assignments as found in many launch windows. The specialized constructors for Column Dialog() mean that you have less flexibility than with New Window(). However, Column Dialog() lets you easily create a launch window.

Help created on 7/12/2018