If there are several possible options, then we need to be clear about what we want. This can be tricky if you don't know what the available options are ahead of time, but luckily most APIs have helpful documentation. Sometimes that's a selection of examples as code snippets, or sometimes you may get a reference list of values that you can call on.

For our milk analogy, the options might look something like this:

my-corner-shop API documentation
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milk options
- size
-- 500ml, 1 litre or 2 litres
- type
-- skimmed, semi-skimmed or whole