Opening Second window in a tab or separate window

Using Xcode 11 for a macOS app I created an app with two window controllers. When first testing the second window would open in Tab of the first window, but along the way I changed something and now the second window opens in its own window.

Not sure what I changed to cause the second window, but for future reference I would like to know how to control this?

Thanks

Since this is an Apple frameworks question, this would be best asked over on Apple developer forums, StackOverflow, etc.. However, your description is pretty vague, and without some more details, it's kind of hard to figure out what you are asking. How are you creating the window controllers? How are you defining the windows (SwiftUI, xibs, etc.)? Is this a document-based application? What changes did you make?

Since I am new to this I was hoping one with experience could easily spot how this is controlled. That is a single setting in IB?

However, I also learning of all the appropriate places to post my questions. I am finding there are many pieces and parts to building an macOS app. However, I am enjoying the learning.

So, I will post this question on the Apple Developers forum with more specifics.

Thanks for the help.