In summary, the MSL has been set in different ways for different parts of the rail industry.
TOCs (both heavy rail and light rail) will be required to operate 40% of normal services on strike days. I suspect that's a reference to the absolute number of schedules, which could mean less than 40% of passenger vehicle-miles operate, as some services may be curtailed (e.g. Waterloo to Exeter services may only run as far as Basingstoke or Salisbury). Operators are free to choose which services to run, but the government's wording suggests they expect them to largely run services during the peak morning and evening hours.
Infrastructure providers such as Network Rail and Rail for London will be required to have "priority routes" as identified in the following map open between 6am and 10pm, including sidings and depots within a 5 mile radius of these lines:
View attachment 146089
There's an exact list
here; it's effectively the same as Network Rail's existing "Key Route Strategy" network (i.e. the lines that were open during the signaller strikes), with a couple of additions.
Obviously the opening hours are considerably longer than what happened during the signaller strikes (generally 7am-7pm) so there should be a much more normal level of service on lines that are open. Wales once again gets a pretty raw deal though, with only the GWML to Cardiff included in the priority routes list.