As a rule of thumb any variable dealing with products that has multiple values should be an array of values:
product_category : ["cat1","cat2","cat3"];
Other variables that do not deal with products can be comma separated:
categoryId : "cat1,cat2,cat3";
However, if you want to report in a specific syntax you can use whatever delimiter you'd like. Let's say you want ">" as your delimiter in your reporting. You can use that as your delimiter instead of a comma (This only applies to non product variables):
categoryId : "cat1>cat2>cat3";
Hope this helps.