Halliday & Matthiessen (2014: 343):
By contrast, in an encoding identifying clause, passive• is more or less as frequent as active, e.g. (‘which is the leader?’—) active Tom is the leader, passive the leader is Tom; but only the active will accommodate a further* agency – we do not say they elected the leader Tom. Hence in an active/passive pair such as (‘who are now the main suppliers?’—) active our company are now the main suppliers, passive the main suppliers are now our company, the agentive form is this decision leaves our company the main suppliers; the passive does not readily expand to this decision leaves the main suppliers our company.
• Note that the active/passive distinction at clause rank is now termed 'operative/receptive'.
* Note that in an encoding clause, the Identifier/Token is the Agent.