account_id – If your application sends segments to a different AWS account, record the ID of the account running your application.
Subsegments:
account_id – If your application accesses resources in a different account, or sends segments to a different account, record the ID of the account that owns the AWS resource that your application accessed.
Segments:
Subsegments:
AWS Documentation: https://docs.aws.amazon.com/xray/latest/devguide/xray-api-segmentdocuments.html