Sequence num can be considered part of the header for many app messages. It’s a control device. I consider it as a session-related tag on a non-session message.
Officially, sequece num is NOT part of the header. Up to FIX.4.4, the header contained three fields: 8 (BeginString), 9 (BodyLength), and 35 (MsgType) tags.
There are several tags about sequence num:
(34) MsgSeqNum Integer message sequence number.
(36) NewSeqNo New sequence number
(43) PossDupFlag Indicates possible retransmission of message with this sequence number
(45) RefSeqNum Reference message sequence number
https://drivewealth-fix-api.readme.io/docs/resend-request-message describes a resend request message i.e. a 35=2 message, containing —
(7) BeginSeqNo and (16) EndSeqNo specifies the requested messages
