Presumably it means the belgian defensive lines are finished before the war breaks out. The original plan was for the Maginot and the belgian line (I forget the name) to form a contiguous line with the Ardennes being lightly covered on account of its supposed impassability for motorised units. When Belgium IOTL became neutral in 1936 the french were left with half a plan and had to rush to meet the germans. It may result in the germans being pinched off à la Blunted Sickle. It may just serve to make the Westfeldzug bloodier for both sides.
That Belgium Fortress line never got a name
Original it was planned to extend the Maginot line to Belgium
then modified to five big Fortress and line of Small Bunkers that connect to Maginot line
But in end only Four of those Fortress were build and additional Bunkers line never build, simply Belgium had not money for that.
instead the pre WWI fortresses of Liege and Namur were reinforced with concrete and better ventilation against Gas Attacks.
To make matter Worst the four Fortress were not equip with needed cannons to support each other, but got short range guns
So the Crews of Fort Tancrémont stand helpless against the Wehrmach Attacks on other three Fortress.
Another issue was that chaotic situation in Fort Eben-Emael that was used by Belgium army as a form of penal institution in 1930s
and litte issue that Commanding officers spoke only French, while solders only understand Netherlands...
Would had help a French-Belgium accord ?
it's depend of Situation and what strategy would be implied
Had the Belgium build there Five fortress with financial help of France
would be issue of there belgium Personal reacting right on situation
if yes Fall Gelb would serious delay by attempt to conquer the Five fortress while all five give each other support fire.
but still we got the Gap in Ardennes with Small Bunker line that would hinder the Wehrmacht, not stop them.
Now those bunker crew can inform High Command, that Wehrmacht try to passt the Ardennes
what was total surprise for Allies as that happened in WW2
Now informed what is happing the French and British could react and move there troops to intercept them better
during Fall Gelb the Allies forces had move into Belgium under assumption the Wehrmacht would redo the Schlieffen Plan of WW1
only to get informed that Wehrmacht force had move true the Ardennes and invaded France while Allies were waiting in Flanders.
But with even with five Fortress and Bunker line, the Wehrmacht is not stupid, the would just adapt there Strategy !
They simply with help of german Boy scouts would look for weak spots in defense line (like OTL)
Also adapt there plans for a "heckling" attack while small force attacks the Fortress from east
another much larger force could move over the Netherlands and attack from behind
Next to that the Germans would R&D for "Bunker buster" weapons used from Bombers