Hi Sam,
If I understand correctly, you are substracting “Items needed” (a string pointing to a linked field) and “Items Owned” (another string pointing to a linked field).
If that is the case, it won’t work, you cannot substract strings even if you use arrayjoin() to make a unique long string with the descriptions.
What you can do is to evaluate the length of those arrayjoins and then, compare them using min() or whatever the arithmetic function. This will tell you if both fields are different in length, but of course you cannot know what are the real differences because of the ordering of the links when they were created.
Another option by using arrayjoin and count the number of colons created in both fields by that function, then compare those numbers. Again this is just approx since you don’t know exactly what fields are compared
I don’t think that this has an easy solution, if you want to compare item by item you cannot do it directly with a function, neither for linked or own fields.
I have a similar case in one of my bases, and I could solve it using a masking function (powers of 2 indicating existence or not of an item, quite convoluted), but that works only if you have a few number of different items
Another option would be creating an automation, fired every time the linked fields are modified, that uses a script task and then program a routine in javascript that can compare value by value. This can be the most powerful solution, however you will need to code.
