What is “Re-man” in freight terms?

Re-man stands for re-manifest.

According to CBSA Memorandum D3-1-1, page 8, #62:
In situations where cargo must be re-manifested (for example, a change in the carrier who is transporting the in bond shipment), a new cargo control document, otherwise known as a re-manifest, must be presented to the CBSA before the goods are allowed to proceed. The information shown on the re-manifest must match the information on the original cargo transmission. Any discrepancies must be approved by the CBSA. The re-manifested cargo control document must have a new cargo control number with the original cargo control number notated in the previous cargo control number field.

For more information refer: http://www.cbsa-asfc.gc.ca/publications/dm-md/d3/d3-1-1-eng.pdf