Это то, что называется путь/направление потока (flow direction/path)

Flow
Path(1 https://www.ar-resources.com/UploadedFiles/YD AND YS SERIES1...

Flow Directions
Design YD (Common Port on Bottom) see figure 5
Plug Down, Left-Hand Port Closed—Flow in
converging service is from right to bottom port and in
diverging service is from bottom to right port.
Plug Up, Right-Hand Port Closed—Flow in
converging service is from left to bottom port and in
diverging service is from bottom to left port.
Intermediate Plug Positions—Flow in
converging service is from both left and right ports to
bottom port, with capacities in proportion to plug
travel. Flow in diverging service is from bottom port
to both left and right ports, with capacities split in
proportion to plug travel.
Design YS (Common Port on Left) see figure 5
Plug Down, Bottom Port Closed—Flow in
converging service is from right to left port and in
diverging service is from left to right port.
Plug Up, Right-Hand Port Closed—Flow in
converging service is from bottom to left port and in
diverging service is from left to bottom port.
Intermediate Plug Positions—Flow in
converging service is from both bottom and right
ports to left port, with capacities in proportion to plug
travel.

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