transmission as in onforwarded ... so a document is created at locality A and then the information in it needs to be forwarded to locality B and/or locality C for approval/denial and further administrative action (including deciding to take no further action)... so the original document stays at locality A and a copy of it is made. The copy is 'transmitted' ... ie handed over in an 'official' way to someone who has authority to carry it to locality B.
These transmission practices happened in the penal admin, and in the civil authorities at least until the mid 1870s in NSW.
So for example, the NSW Governor in 1810-1821 era was Lachlan Macquarie. His admin would have needed to send copy of his general orders to penal settlements including Wellington NSW, Newcastle NSW, Port Macquarie NSW, and various localities in Van Diemens Land, and Norfolk Island. He also had communications to transmit to new settlements in New Zealand. Those copies were 'transmitted'. Rev Samuel Marsden held both religious and civil admin positions. So one of the civil admin positions he held meant that he needed to receive transmitted penal records. And one of his religious positions meant that he needed to receive transmitted clergy records.
That is the briefest explanation I can give. The thesis could run into several hundred pages.
OOPS
Add, so sometimes the only extant record is the original. Other times NO extant record exists, but a log in letters IN shows it was received at the 'end' locality. NSW State Archives holds 'everything' from that era. People who live in Sydney and who can get to the State Library and or the Archives are made very welcome. There is plenty of disability access at both localities. The staff are knowledgeable, helpful and keen to assist any enquiry. I know this from first hand experience and also from my own ancient rellies, retired Archivists.
JM