In Appian we can use a records fields by square bracket notation or index or dot notation.
what to use where ,what is the best practices , i heard indexing is the best.in some places its not working ,i don't have clear understating
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Correct. The difference between a reference and the actual value is a very basic programming concept and you will find a lot of explanations in the internet.
I think the practical upshot most of the time is that the square brackets are far more susceptible to pink box crashes. I've not seen a tremendous problem with using index() all the time.