In a portal you can use Portal filters to make a list of related records appear depending on 1 or many fields values.
Say you have Articles and want to see only part of those articles in a portal
Here is a good article to read how you handle that
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You can also download these instructional files.
The first has several different working examples of exactly what you asked for, hierarchical conditional value lists. The second has working examples of a filtered portal that updates as you type.
Both files come with detailed documentation on how they are set up and how they work.