See Sections 1 and 3 of the ECAC Airspace Planning manual which respectively provide General Guidelines and guidelines relating to Airspace Restrictions/Reservations in support of the FUA concept.
6.4.3 SECTORS
From a design perspective, the sectorisation of a Terminal Airspace is one of the most common ways in which to distribute workload between controllers so as to ensure the safe and efficient management of air traffic within the airspace volume. Whether Sectorisation is necessary is decided – almost exclusively – on the basis of ATC workload which may impact upon safety. Because the frequency and number of air traffic movements constitutes one of the main factors affecting ATC workload, the importance of the selection of a realistic traffic sample and identification of the predominant runway in use cannot be over-stressed. As discussed in Chapter 3, the selected traffic sample is one of the major assumptions of the design process. Once it has been properly analysed (as regards time and geographic distribution), it is assigned to the modified or new Terminal routes which have been designed. Qualitative assessment of the traffic sample supported by Airspace Modelling are common methods used to identify the need for Sectorisation.
Comment: Is there a generic rule of thumb that allows designers to estimate the need for Sectorisation? Airspace designers will be aware that, for the most part, States do not publish capacity figures for Terminal Airspace (or TMA) sectors. There could be several explanations for this e.g. (i) it is too difficult to calculate; or (ii) capacity limitations are ‘hidden’ by published airport capacity and/or en-route sector capacity; or (iii) capacity figures are not calculated for Terminal Airspace or its sectors. There is a fourth possibility – which is unlikely – and that is that there are no capacity problems in Terminal Airspace sectors in ECAC. Whatever the reason, there appears to be agreement on the fact that capacity is difficult to estimate in a Terminal Airspace – perhaps because it is sandwiched between En route and the airport.
In order to appreciate the complexity of determining capacity of a Terminal Airspace volume (or sector), it is worth mentioning the variety of factors which affect the number of aircraft that can be handled by a single controller in a given time period. Importantly, none of these factors can be viewed in isolation. Each factor is a ‘variable’ in the overall capacity ‘equation.
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Design of Terminal routes. The more segregated the routes both vertically and laterally, the less the ‘active’ the workload of the controller;

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Use of designated arrival and departure routes such as SIDs/STARs. Generally, the greater the number of published routes, the less RTF required (Note, however, that an excessive number of SIDs/STARs can create a high pilot workload or introduce errors).

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The accuracy of the navigation performance of aircraft operating on designated routes. The greater the accuracy, the less the need for controller intervention.

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Phase of flight. Generally, arrivals are more labour intensive than departing flights especially if extensive use is made of tactical routeing as opposed to designated routes such as STARs.

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The complexity of the instrument approach procedure : especially in terrain rich areas or for reasons of environmental mitigation, the Radar monitoring workload can be high with respect to complex manoeuvres.

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The altitude of the airport, ambient temperature and airport infrastructure affect runway occupancy and in-trail spacing interval. At ‘hot and high’ airports, holding may be required to compensate for any of these factors – which is work intensive.
　
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