Federations, trade bodies and public authorities
The buyer type with no clean register anywhere, which is exactly why it stays underworked by every agency including the ones that serve it well. Sector federations, interbranch organizations, professional unions and local authorities each buy communications, each on a mandate cycle, and each are reached by name and by mandate rather than by list.
Who signs: the director general, the communications lead, the elected president, the cabinet director on the public side.