The IRAF equation for error estimation is an approximation for SNR>>1. If someone doesn't trust that this is valid for all their data, it is better to manually calculate the errors using the SNR (see the first link for a brief explanation and the equations).
Can I quote the article (second link) as a valid reference for how IRAF calculates the final errors in a CCD photometry process*?
* Such as the one described in A User's Guide to Stellar CCD Photometry with IRAF by Massey & Davis (1992)
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