If it says ‘A or B’, this means that it is possible to have both A and B, but you don’t automatically have both of them in. The situation that ‘A or B’ rules out is the situation where neither of them are in.
If it says ‘A or B but not both’, this means that one of them is in, but you can’t have both in and you can’t have both out.
On page 183, when it says “If J or M, then F”, this is a conditional rule: If we have J, or M, then F is in as well. But if we have J, this rule tells us nothing about whether or not we have M, and likewise if have M, this rule tells us nothing about whether or not we have J.
When it says “Either H or I, but not both”, this means that either H is in, or I is in, but you can’t have both H and I in, and you can’t have both H and I out.
Hope that helps 🙂