Complex sentence: If the master (the captain) issues a B/L to a shipper, he will be acting as an agent for the carrier, who will be either the shipowner (time or voyage) or the charterer (demise).
Split into simpler sentences: If the master (the captain) issues a B/L to a shipper, he will be acting as an agent for the carrier. The "carrier" will be the shipowner (time or voyage) or the charterer (demise).