Hi,
I'd say that "illiquidity premium" is a general term, while the matching adjustment is what this concept is called under SII. So these are not really two separate things.
For example, SII is very prescriptive regarding WHEN and HOW you're allowed to apply the MA. You need to demonstrate that certain eligibility criteria are satisfied and, if so, the calculation steps are pretty much laid out in the Directive. First, project the asset cash flows (from an eligible pool of assets) which closely match the best-estimate liabilities, work out the gross redemption yield and then deduct the portion of this yield which corresponds to the risks of default and credit downgrade. What is left in excess of the RFR is a spread we call the MA. In the de-risking process, you'd rely on default probabilities and other relevant assumptions published by EIOPA.
However, allowance for illiquidity premium can be also made for other reporting purposes (e.g. under IFRS) and here there would normally be more freedom regarding the calculation methodology. Conceptually, you'd do something broadly similar to the MA calculation, but the approach (and the result) would not necessarily be the same.
In summary, under SII you can take credit for the MA but NOT for both the MA and illiquidity premium, as you wrote in your question. An alternative uplift to the basic risk-free rates in the SII world is something called the volatility adjustment (VA). However, even here you can either apply the MA (if you're allowed to) or the VA (if you you're not able to apply the MA and you demonstrate that certain conditions are met).
Regards,
Mateusz
Last edited: Mar 17, 2019