Halliday & Matthiessen (1999: 122):
Thus sequences are formed by binary logico-semantic relations which may relate either single figures or sequences of figures [i.e. internal nesting]. Certain relations may impose constraints on the phenomena being related; for example, a projecting relation can only obtain when the first figure is one of sensing or saying. But the range of different logico-semantic relations is highly varied, so that constraints tend to be specific to particular subtypes.