The practise many GX4000 cartrige game developpers had to directly put an almost unchanged Game form the CPC OLD library into a Cartridge.
Some time such games even had to be lighted from some features (graphical page) as the 128Ko only Cartridge was a bit short of Bytes compared to a 3" Disk.
Of course this didn't helped the Plus range at all and was almost as shamefull as the Speccy Port technic.
This was a cheap and fast way to shortly release games.
Another problem came from the only 64ko Ram of the Plus standard : 464+ and GX4000.
As a result, implementing Plus features on existing games could be impossible without massive reprogramming if such games already used well the full 64Ko Ram.
Yet a 128Ko Ram would allows additions of Hard Sprites or DMA sounds more easily on a 64Ko ram using old game.
Most Cartridge games were "Almost Direct Disk to Cartridge Port" as some few extra PLUS features where still implemented :
But it was better than nothing.
Most of those games were known as good games ond the CPC Old.
The bet was that it would still make good games on Plus standard.
Yet peoples saw no interest to buy an expensive Cartridge with so few adventages, as they often had the Cracked Disk version.
Most games with OLD and PLUS version are "Almost Direct Disk to Cartridge Port".
Yet the Extra PLUS feature could vary a lot.
Batman and Barbarian 2 could have make great use of 2 button padles and a 4096 colour palette...
It is nowadays a common feature to modify existing Old CPC games into "Almost Direct Disk to Cartridge Port", just by modifying the Colours palette to match the Plus capability.
Yet heavier modifications can be seen too.
As most cartridges were the original Games release from the GX4000 era, it was mostly applied to games, yet the crack of the ACID chip and production of modern era Cartridges (or multi-ROM cartridges) may lead to Direct Disk to Cartridge Port applied to any knid of softwares.