В текста Ви стои думата substructure, която е всъщност част от цялостното понятие.
Понятието е свързано не толкова с териториалното деление на Южна Африка, колкото с местното самоуправление. Намерих един текст, който е доста стар (1995 г.), но дава някаква идея. Transitional metropolitan substructures са свързани с Transitional metropolitan councils, които администрират тези преходни единици:
Transitional Metropolitan Councils (TMCs) are forms of local government, new in South Africa, which must administer large and complex urban areas with multiple pre-existing local authorities. Those which have been promulgated thus far, or are about to be formally established, are Greater Pretoria, Greater Johannesburg, NorthEast Rand (Midrand/Kempton Park/Ivory Park/Tembisa, etc.), Vaal Metro, Greater Durban and Greater Cape Town. These proclamations cover the pre-interim period, and it is quite possible that the boundaries could change before the local government elections.
Each TMC must have two or more internal TMSSs (Transitional Metropolitan Substructures), which like the TMCs will be elected in November but are presently appointed.
There are two very different models for TMSSs:
• a grouping of pre-existing local authorities which merge into a TMSS but retain the character of previous boundaries, such as the grouping of Akasia and Soshanguve into the Northern Substructure of Greater Pretoria; and
• a redrawing of internal boundaries with less respect for pre-existing local authority boundaries, as in Greater Johannesburg which has seen the extinction of most previous boundaries in toto.

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Note added at 1 hr (2012-01-14 11:36:52 GMT)
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Не знам дали "градска" е най-доброто предложение, но пък "столична" някак си също не става, защото столицата на ЮАР е една.

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Note added at 1 hr (2012-01-14 11:55:42 GMT)
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Освен това, вероятно с времето тези TMSSs доста са еволюирали, след като град Странд има своя собствена, а той има едва около 50 хиляди жители.