The Related subtype aggregates multiple related entities. The message consists of a root (the first entity) which references subsequent inline entities. Message entities should be referenced by their Content-ID header. The syntax of a reference is unspecified and is instead dictated by the encoding or protocol used in the entity.
RFC consulted during implementation:
RFC-2387 The MIME Multipart/Related Content-type
The first entity added becomes the root object. The related message type is set to the value of the root object media type.
# File lib/mime/composite_media.rb, line 239 def add entity unless type.include? '; type=' root_type = entity.type.partition(';').first # omit parameters self.type = append_field_params(type, 'type' => root_type) end super end