I was confused by your use of ‘flags’. It is perhaps more useful to read first about format(). Even if you are more likely to use the dot-operator version, the original operator helps show how different source data types affect the result.
Here, you are referring without clarification_ to the formatting of lists which would have been a better question. I don’t mean that as rebuke … but garbage-in-garbage out. There are no format()-specific designators. What happens/what’s needed as input is entirely dependent on the source data type.
With a list, thus in Tinderbox a List-type or Set-type, the operator takes a single argument the ‘formatString’: this is an unevaluated string.formatString is the string used in the function’s output to replace the semicolon in the stored version of a list. Thus ", " places a comma+space between each term, whilst “\n” inserts a line break between each term etc. So " banana " would place space+banana+space between teach item, etc.
I’m confused as to how this is not clear in the existing documentation, though it seems there is potentially an issue over the user’s vs. author’s starting PoV. I’m open to suggestions as to how to make this better.
For specific data types, such as ‘Date’ type, there are a number of special format strings that can be used to give precise versions of formatString when rendering the source item(s).