A manifest, customs manifest or cargo document is a document listing the cargo, passengers, and crew of a ship, aircraft, or vehicle, for the use of customs and other officials. Where such a list is limited to identifying passengers, it is a passenger manifest or passenger list or bag manifest; conversely, a list limited to identifying cargo is a cargo manifest or cargo list. The manifest may be used by people having an interest in the transport to ensure that passengers and cargo listed as
having been placed on board the transport at the beginning of its passage continue to be on board when it arrives at its destination.

This document, made up generally by the ship's broker, from the contents of the bills of lading, contains a specification of the nature and quantity of the cargo laden, and is generally attested officially, and in some countries notarially. The prize laws seldom mention this paper; nor is it general; but yet of essential importance in case of search, as well for belligerents, as for neutrals in procuring a speedy dismissal. It is usual to require it at the custom house. (p301)

In this way, a cargo manifest is like a passport except that it is used for goods instead of (or in addition to) persons; the manifest is evidence of the nationality of the goods, the absence of contraband, and that property belonging to belligerents is not laden on board of the vessel.

Vessels are under no legal obligation to carry a manifest and, indeed, it is only necessary for neutral vessels in a time of war.

Cargo manifest versus bill of lading

A cargo manifest and a bill of lading may carry similar information and the concepts are not always clearly distinguished. In some cases, a single document may serve both purposes. In general, a bill of lading serves as a legal instrument focusing on and documenting such issues as ownership, whereas a cargo manifest is often more concerned with physical aspects of the cargo, such as weight and size. When the cargo is being shipped by several shipping companies on the same vessel, there will usually be separate bills of lading for each company, but only a single consolidated cargo manifest. On the other hand, if the cargo contains dangerous goods, there may be a separate dangerous cargo manifest.

Source: https://en.wikipedia.org/wiki/Manifest_(transportation)