A list can be anything. List of numbers, letters, elements ex… Some nodes can only except elements, strings or numbers.
List.FilterByBoolMask is just going to
In: Pass the items in the list input with a corresponding True values from mask.
Out: Pass the items in the list input with a corresponding False values from mask.
String.Contains is going to create a True / False list based on whether or no the items from str contain the item in Search For.
You took a parameter from and checked if it had Oxygen. The parameter with oxygen are True and the ones without are False. If you place the Elements List, Parameter value list and the Fattened list next to each other they should look like this.
0 Element Oxygen True
1 Element Air False
2 Element Oxygen True
3 Element Oxygen True
4 Element Air False
5 Element Oxygen True