Halliday & Matthiessen (2014: 261-2):
In ‘relational’ clauses, there are two parts to the ‘being’: something is said to ‘be’ something else. In other words, a relationship of being is set up between two separate entities. This means that in a ‘relational’ clause in English, there are always two inherent participants – two ‘be-ers’. In contrast, the general classes of ‘material’ and ‘mental’ clauses have only one inherent participant (the Actor and the Senser, respectively). Thus, while we can have a ‘material’ clause with one participant such as she was walking or she was walking into the room, we cannot have a ‘relational’ clause such as she was, with only one participant; we have to have two: she was in the room. Similarly, a ‘mental’ clause with one participant such as she rejoiced is possible; but the nearest ‘relational’ equivalent must have two participants – she was happy, not she was.
Note that attributive clauses with qualitative Processes, such as this stinks, manifest only one participant, unless the Attribute is interpreted as conflated with the Process; see Halliday & Matthiessen (2014: 271).