[IanCal]

> The subsection headed ‘User numbers’ (which begins at paragraph 5.7) explains when a service is to be treated as having more than a particular number of monthly active United Kingdom users for those measures which apply in relation to services of a certain size, and how to calculate the number of monthly active United Kingdom users. The definition of ‘large service’ is included in the definitions section in Section 5 of this document.

That section details how to calculate the figures, because they're relevant for sections like CSAM scanning

> Services that are at high risk of imagebased CSAM and (a) have more than 700,000 monthly active United Kingdom users or (b) are file-storage and file-sharing services.