What is this Carter BBD port for?

The port fittings in the pictured carburetor are used as follows:

1. A full flow manifold vacuum port. Used on ~1979 and later for the brake booster due to other vacuum fitting complexities. Not needed for 1967. Cap if not used.
2. Metered vacuum to the choke pull-off as shown.
3. A full flow manifold vacuum port. Connect to the PCV valve if not connected elsewhere. Otherwise capped.
4. Vacuum for the air cleaner flap(s). If the air cleaner has a temperature sensor, connect to it; if no sensor, connect to the hose(s) that leads to the flap actuator(s). 1967 models were not originally equipped with exhaust manifold pre-heat and air flaps in the air cleaner. Likely a 1970 or later air cleaner has been installed.
5. Ported vacuum for the distributor vacuum advance.
6. Bowl vent. For 1967, this was not a tube like this but a lift valve that opened to the atmosphere. On my carburetors that are open fittings, I fit a small open mesh screen filter. If using a charcoal canister, connect to its vent port. If using a 3-port 1970-72 valve cover breather, connect to the 3/8" port on it. Do not cap off. It will hamper fuel flow if capped.
7. If this is smaller than 3/8", it is the purge vacuum for an evaporative charcoal canister on 1973 and later. 1967 models were not originally equipped with canisters. If one is installed and plumbed, connect to its purge port. If it is 3/8"-11/32", it is the PCV port. Connect to the PCV valve in this case. Otherwise capped.
8. Venturi vacuum sense port for EGR, 1973 and later, Not used for 1967.

Port 4 is a good source for your vacuum gauge when testing. The idle increases when the port is opened due to the increased air flow. This may not indicate any issue, but could indicate the idle mixture is otherwise a little rich. Set the idle settings with your vacuum gauge connected, and it will be essentially the same when capped, or nearly the same when connected to the air cleaner assuming a good sensor, if used, and actuator.

This clone carburetor looks like it's modeled after the late 70s Carter, and intended for semi-universal use. If they offer an earlier clone style, it likely has less fittings and may be jetted a bit richer, but this should be workable on most stock or near stock 318s.

The 318 was the base engine in many C-bodies, particularly low line models, polysphere through 1966 in the US, and 1967 in Canada, and LA thereafter through 1970. The 360 became the base engine in many after that. For tuning the LA 318 in your 1967 Canadian Monaco, check against the 1967 US LA 318 settings, although the polysphere settings are likely close enough.