Organizational Structure of Intelligence Services
In democratic states, any organizational model, division of responsibilities or coordination model has an illustrative example. Each country has for its model of intelligence and security services historical reasons, tradition and has to solve the advantages and disadvantages of the structure during the operation. Without taking into account specific historical reasons, it is not possible to say that this or that existing model is good, better or worse. The international law (including requirements of EU, Council of Europe and NATO) does not give any clue or recommendation for intelligence services' structure; it is the sovereign right of each country.
One of the rules might be that one institution has one specific focus. Democratic states usually have separate internal and external intelligence services. The reason for this arrangement is their different objective and more importantly, the fact that the operations within their own territory and abroad follow different rules and have to comply with different legal system.
Source: Petr Zeman - lectures, trainings and public speeches