SoA classification level

Answer:
Because SoA has many information about how the organization approaches information security, it is a sensitive document and access to it should be restricted to personnel that requires it to perform their activities (e.g., top and middle management, and the security officer) and in most cases this does not cover all employees. Considering that, in most classification frameworks the lowest level which has this kind of restriction is the "Restricted" level, but you have to check your own framework to confirm that. You should avoid to use the highest classification level you have because in most scenarios the highest classification level will demand controls that will be too much to protect in the SoA.

For further information see: Information classification according to ISO 27001 https://advisera.com/27001academy/blog/2014/05/12/information-classification-according-to-iso-27001/