Halliday & Matthiessen (2014: 328):
Angle is related either to
(i) the Sayer of a ‘verbal’ clause, with the sense of ‘as … says’ or
(ii) to the Senser of a ‘mental’ clause, with the sense of ‘as … thinks’.
We can call type (i) ‘source’ since it is used to represent the source information …
We can call type (ii) ‘viewpoint’ since it is used to represent the information given by the clause from somebody’s viewpoint …