I asked on the Slack channel Any specific reason why the downstream is set to 200 in the GenomicFeatures::promoters function? Herve Pages pointed out This is just a default value that was set to be consistent with GenomicFeatures::getPromoterSeq(). Paul Shannon is the original author of getPromoterSeq(). I don't know if Paul is on this slack but if you ask on our support site you might get a response from him.

I would like to know if there is more to it

I don't know the exact origin of this, but it's a common definition of promoter. Sometimes it is -1kb +200bp instead.

You can see the motivation though, analyzing the sequence for conservation and TFBS motifs, it makes sense to include downstream nucleotides in the sequence that is defined as initiating transcription.

See: https://www.nature.com/articles/srep25164

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