It’s an implementation of ODK[1] extensions to the XForms specification[2]. The specific implementation is Enketo[3], which I was brought on to ODK to help maintain. Not exactly the spec I’d go with greenfield, but it’s been successfully in use for a decade with great impact.

1: https://getodk.org/

2: https://getodk.github.io/xforms-spec/

3: https://enketo.org/