ACI is the Canada-bound counterpart of ACE, with the same division of labour: the carrier reports the truck and its cargo, and the customs broker requests release of the goods, usually through PARS. Each shipment is identified by a cargo control number that begins with the carrier's CBSA-issued carrier code.
The two governments do not ask for the same things, and treating the two filings as one is how mistakes happen. For a carrier running both directions it means two filings, two portals and two rule sets covering the same truck on the same day — which is why border work tends to collect around one person in the office, and slows down when that person is away.