Yes, you got the lingo right. 
However, what you want can't be done in the exact way you want. Each parameter in Crystal Reports must be entered by the user and they don't have any options for using on or the other. However, you could get around this problem by making one of the parameters ask the user for the type of date they want to enter (give it two defalut values of Open or Closed) and the second parameter is the date.
In your record selection fomrula, you will have to match the date to the proper field based upon what they entered in the first parameter. It would go something like this:
({?Type} = "Open" AND {?Date} = {table.OpenDate})OR
({?Type} = "Closed" AND {?Date} = {table.CloseDate})
As you can see, the date that is used for comparison is based upon the Type parameter that you created with the two default values.
I think this will be what you need. If not, feel free to post a follow up with more details.
My Encyclopedia book gives you tutorials for learning how to create parameters and walks you through the steps of using them in your record selection formulas. You can find out more about my books at
Amazon.com or reading the
Crystal Reports eBooks online.
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This is exactly what I needed. I'm used to finding similar workarounds in Access and other applications, so it makes perfect sense to me that this is the way to handle it. 
