Classification of Intelligence Services According to the Method of Information Gathering
The intelligence services can also be classified according to the means by which the service gathers information. There are services which use more methods of information gathering and there are specialized intelligence services which focus "just" on an exclusive method. It is for instance the British Government Communications Headquarters (GCHQ), which uses only the so-called communications intelligence, i.e. interception of communications.
Source: Petr Zeman - lectures, trainings and public speeches