String.Contains node will get you started. You can add “RM” string value to search for. It will give you booleans list and then finally you use filter by bool mask node to filter. Make sure you set lacing to longest for String.Contains node as your list contains sublists.
Thank you for taking a look, using the web as it was intended.
Following-up:
What I am trying to do is omit the entire sublist if a specific index (#4) in the sublist contains a specific value (NULL). (a slight change from when I was looking for the string “BM”).
Why do I need to do this? It will eliminate rooms that are not in the model from the list of room properties since node #4 is room area which is null for rooms that are not placed.