Laerte Junior explains how dynamic parameters work in Powershell:
Have you ever been in a situation that you want to call a cmdlet or a function with a parameter that depends on a conditional criteria that is available as a list? In this article I will show a technique where you can use PowerShell Dynamic Parameters to assist the user with parameter values.
In the documentation of Dynamic Parameters found at about_Functions_Advanced_Parameters in
get-helpit is defined as “parameters of a cmdlet, function, or script that are available only under certain conditions.” And can be created so that appears “only when another parameter is used in the function command or when another parameter has a certain value.” So, we can say that PowerShell Dynamic Parameters are used when the result of a parameter depends on the previous parameter.
Click through for examples.