For the select type, I believe that’s wrong description, we will fix, for now it’s possible to pick only 1 value.
For relation, let me explain:
Single Type
You can still choose multiple items (as many), however system will always return just one randomly, so if there’s 2 items selected, on each request system will choose on of those 2 and return as single item.
Multiple type
You can choose as many, and items will be returned as list (array)