Section 10 of the EI specification describes Market Context indicating that both parties in
a transaction can have a "market context". Furthermore, the narrative framework (section 5.3)
describes VEN's querying VTNs for their market context. However, when looking at the schema, the only
two places where market context appears in the payload is as follows:
1) Various eiOpt payloads, in both the requests and responses. We assume that the responses are just a
mirror of the requests marketContext. If not, this needs to be described.
2) In eiRequestStatusPayload and as part of eiSentStatusPayload eiEventDescriptor.