Discounting

The notes in Chapter 9 section 4, page 8 state the following:

"For statutory returns the SFCR liabilities may be discounted following Solvency ll rules.

Explicit discounting may be applied to categories of unpaid claims reserves at the end of an accounting period. This is applicable where the average expected period to final settlement from the end of that accounting period for the category of claims in question, weighted on the basis of expected gross claims, is greater than four years. This may be particularly relevant for annuities for non-life business (PPOs in the UK).

An insurer should only consider discounting if there are adequate data available to construct a reliable model of the rate of claims settlement. The insurer should take a cautious approach in assessing the amount and timing of future cashflows to provide greater confidence in the adequacy of figures emerging from the model."

I'm trying to figure out where the rules or guidance on the expected period to settlement being greater than 4 years comes from? Is it Solvency II, IFRS 17 or just guidance? I can't seem to find it anywhere in Solvency II (in fact most presentations say discounting is mandatory, as did a work colleague) or IFRS 17 (where the notes later state that all cashflows must be discounted under the GMM / BBA). Please can you direct me to where this rule / guidance comes from. Is it in the delegated acts or guidance and which Basis is it to be applied to (Solvency II or IFRS 17)? Or is it only for the SFCR?