I am not permitted to provide an interpretation, but I can provide some ideas which may help.

A single Regulation cannot be read in insolation, to be honest. In this case, it's under Regulation Group 421.

If you look at the 2018 version of the Regulation:

421.1.7 Arc fault detection devices conforming to BS EN 62606 are recommended as a means of providing additional protection against fire caused by arc faults in AC final circuits.

Viewed on its own, with the proposed interpretation which in terms of the word 'recommended' ought NOT to change, because the source BSI document for drafting standards is the same, if you didn't fit them to AC circuits to BS 7671:2018, what did you do?

Surely the other "several possibilities" alluded to are in Regulations 421.1 to 421.6, and other parts of Chapter 42 as appropriate? It's the only way that Regulation 421.1.7 in BS 7671:2018 makes sense not being worded "shall".

Put another way, if we are not considering an installation in any of the bulleted premises, we can effectively grey out the related words for those premises, and view Regulation 421.1.7 in BS 7671:2018+A2:2022 as only a slightly modified version of the one in BS 7671:2018, which is a heck of a lot clearer and accedes to the limitations in the scope of BS EN 62606 in terms of ratings etc.:

421.1.7 Arc fault detection devices (AFDD) conforming to BS EN 62606 shall be provided for single-phase AC final circuits supplying socket-outlets with a rated current not exceeding 32 A in:

• Higher Risk Residential Buildings (HRRB)
• Houses in Multiple Occupation (HMO)
• Purpose-built student accommodation
• Care homes.

NOTE 1: Higher Risk Residential Buildings are assumed to be residential buildings over 18 m in height or in excess of six storeys, whichever is met first. It is anticipated that in many areas higher risk residential buildings will be defined in legislation which can be subject to change over time, as well as in risk management procedures adopted by fire and rescue services. Current legislation should be applied.

For all other premises, the use of AFDDs conforming to BS EN 62606 is recommended for single-phase AC final circuits supplying socket-outlets not exceeding 32 A.

Where used, AFDDs shall be placed at the origin of the circuit to be protected. The use of AFDDs does not obviate the need to apply one or more measures provided in other clauses in BS 7671.

So, if we thought it was optional in 2018, perhaps it's optional now (or was it actually optional all along ...)?

I am not permitted to provide an interpretation, but I can provide some ideas which may help.

A single Regulation cannot be read in insolation, to be honest. In this case, it's under Regulation Group 421.

If you look at the 2018 version of the Regulation:

421.1.7 Arc fault detection devices conforming to BS EN 62606 are recommended as a means of providing additional protection against fire caused by arc faults in AC final circuits.

Viewed on its own, with the proposed interpretation which in terms of the word 'recommended' ought NOT to change, because the source BSI document for drafting standards is the same, if you didn't fit them to AC circuits to BS 7671:2018, what did you do?

Surely the other "several possibilities" alluded to are in Regulations 421.1 to 421.6, and other parts of Chapter 42 as appropriate? It's the only way that Regulation 421.1.7 in BS 7671:2018 makes sense not being worded "shall".

Put another way, if we are not considering an installation in any of the bulleted premises, we can effectively grey out the related words for those premises, and view Regulation 421.1.7 in BS 7671:2018+A2:2022 as only a slightly modified version of the one in BS 7671:2018, which is a heck of a lot clearer and accedes to the limitations in the scope of BS EN 62606 in terms of ratings etc.:

421.1.7 Arc fault detection devices (AFDD) conforming to BS EN 62606 shall be provided for single-phase AC final circuits supplying socket-outlets with a rated current not exceeding 32 A in:

• Higher Risk Residential Buildings (HRRB)
• Houses in Multiple Occupation (HMO)
• Purpose-built student accommodation
• Care homes.

NOTE 1: Higher Risk Residential Buildings are assumed to be residential buildings over 18 m in height or in excess of six storeys, whichever is met first. It is anticipated that in many areas higher risk residential buildings will be defined in legislation which can be subject to change over time, as well as in risk management procedures adopted by fire and rescue services. Current legislation should be applied.

For all other premises, the use of AFDDs conforming to BS EN 62606 is recommended for single-phase AC final circuits supplying socket-outlets not exceeding 32 A.

Where used, AFDDs shall be placed at the origin of the circuit to be protected. The use of AFDDs does not obviate the need to apply one or more measures provided in other clauses in BS 7671.

So, if we thought it was optional in 2018, perhaps it's optional now (or was it actually optional all along ...)?