what is it

so its been bugging me for a while but on my 85 dx cc,
the fuel system is a mess..
over the years looks like its been messed with.
there is a white/yellow plastic box which i can see but cannot touch loose above the fuel pump to one side of the tank, the hose clips are all wrong sixes and generally rough but i cant get to it without removing tanks by the look of it and i dont even know if its suppose to be there or someone may have swapped parts out over the years as seems to be with the rest of the car.
would this be the cause of my strange starting issue in where one time its on the button then next time it will crank over & over and eventually fire up rough or, turn ignition off/ on then fires straight up
1. do i need it
2.wheres it suppose to be mounted
3. hows it plumbed
 

Might be better to post up a picture of what you have fitted then we can see if it's wrong…

This might help.
For cars with the 55l fuel tank (space saver spare wheel)

https://volkswagen.7zap.com/en/rdw/golf+cabriolet/goc/1985-44/2/201-69020/

Gravity valve??  if thats the correct term and soft tops have them

From the sounds of it, that is the Filter/swirlpot/sump that the in-tank pumps to so that the main tank can suck it's fuel from.



It is attached by a clip at the bottom, and that is a pretty heavy duty clip to the lip of the tank.

swirl pot not a gravity valve

fuel filter is attached to the front
on the air filter housing not at the
rear