CASE when statements using null

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Ok, at least in my head this is hard to explain.

I want to look at two columns. And then in a third column return a label.

So, the two columns are Relo amount and Sign on bonus amount. I want to return a label based on if there is anything in each column. So if it's just relo, the label should be relo. If it is just sign on bonus then it should be Sign on bonus. If it's both, I want both and if it's none I want none.

I've been trying to use IF NOT NULL in my formula but it doesn't seem to like that. Is there another way to basically see if there is anything in a column and that means it's true?