Do you agree with this?

[Sandro Baptista]

Yep!
It is used when you have capacity control!
Low capacity, oil and refrigerant goes throughout small pipe. Oil in large trap serve as restriction.
High capacity, oil is blown from large trap and goes throughout large pipe.

That is at I think...the principle is universal for all systems. The problem is that the trap on the small riser it shouldn't exist...I mean this riser shall not be sealed!!

[Aik]

Yes Sandro, you are right, but if you look on reducing coupling before large trap, you'll see that oil don't flow to the large trap when low capacity (it's wrong reducing coupling). If you install small riser in top of general suction tube then on low capacities oil couldn't lift until capacity not rise. If you use additional small oil trap then oil will lift independently on capacity control.

[Sandro Baptista]

If you install small riser in top of general suction tube then on low capacities oil couldn't lift until capacity not rise.

The oil is already entrained on the vapor-liquid R717. The P trap is only to oblige the fluids go by the small riser, after the P trap create an oil obstruction.
So when the liquid-vapor R717 up by the small riser will also carryover the oil. If not oil would accumulate further on the horizontal tube below and will promote even more oil entrainment on the R717.