Always a good idea to use the search engine for questions like this, as there have been specific threads on exactly this topic.
From memory - (and I've said this before) there isn't one. ISO only has one for "Flowchart symbols and their use in micrographics". I know Standards Australia has, or certainly had, one for flow charts, so other standards bodies may have similar guides.
These are only guides. Use whatever is easiest, e.g. WORD has a standard set of flowchart symbols to use (and do) or if you have a flowcharting computer program then that'll have a std set.
If you're looking to buy a flowchart program, check out past threads, or post in the quality software section.
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