As I recently learned in John Swinbank's review of PhotoCalib, the DPDD says this about "Err" and "Sigma":
For all catalog data, we will employ a convention where estimates of standard errors have the suffix Err, while the estimates of inherent widths of distribution (or functions in general) have the suffix Sigma
On the other hand, we use Sigma in many places where we really mean Err. For example, our source tables are littered with _fluxSigma and _xySigma, which are documented as "1-sigma uncertainty". Our database schema is mixed in this regard as well (e.g. raSigma, an "uncertainty" and uPSFluxSigma a "Standard deviation of the distribution" in the DiaSource table).
This RFC is to gather consensus about how best to sanitize our usage of these terms in the stack and related software and interfaces. The concrete propose here is to replace all incorrect uses of Sigma with Err, and vice versa in all code below lsst_distrib and in cat.