• When you create a segmentation in Marketo, there is always a default segment that cannot be moved, edited, or deleted. 
• People will qualify for each segment in the order defined. 
• The segments are mutually exclusive.
• A person can be a member of only one Segment at a time.
• When a person qualifies for two segments, they will belong only to the first one in the list.
  If a person does not qualify for any Segment, they will become a member of the Default Segment.

I hope this clarifies your question. I would recommend creating a test segmentation so you can see how it works in practice - sometimes it's hard to grasp these concepts until you get your hands on the tools.

 

You can also read more about segment priority here: https://experienceleague.adobe.com/docs/marketo/using/product-docs/personalization/segmentation-and-...