Fire Extinguishers and the Blue Book: Motorsport UK has replied to a request for clarification:
“The minimum capacities for Small, Medium and Large extinguishers referenced in K3.1.2 are detailed in Table 3 on Page 176. For Circuit Racing 3.1.2(a) applies which is a minimum capacity of 2.25 litres AFFF. These minimum capacities apply to non-homologated extinguishers. For FIA homologated plumbed-in extinguishers (mandatory from 2022) the minimum capacity is defined by each individual system’s FIA homologation.
Hope this helps.
Michael Duncan, Technical Manager, Motorsport UK https://www.motorsportuk.org/ “