Rockwell Automation Logix5000 Controllers Function Block Diagram Programming Manual - Manual (Page 15)

Program a Function Block Diagram Chapter 1
To identify which block to execute first, mark the input wire that creates the loop
(the feedback wire) with the Assume Data Available indicator. In this example,
block 1 uses the output from block 3 that was produced in the previous execution
of the routine.
The Assume Data Available indicator defines the data flow within the loop. The
arrow indicates that the data serves as input to the first block in the loop.
Do not mark all the wires of a loop with the Assume Data Available indicator.
This example is OK. This example is not OK.
The Assume Data Available indicator defines the data flow within the loop. The controller cannot resolve the loop because all the wires use the Assume Data
Available indicator.
Resolve data flow between two blocks
If you use two or more wires to connect two blocks, use the same data flow
indicators for all of the wires between the two blocks.
Rockwell Automation Publication 1756-PM009E-EN-P - October 2014 15