parallel processing - How does SSIS decide which task to execute next if more than one is available -

For example, if I add four (empty) script actions to A1, A2, B1 and B2, where A2 has an obligation,

After B1, there is an obstacle after A1 and B2, but there are obstacles in A1 and B1, then how will the work run?

When I try it on A1, B1A2, then B2? This question with curiosity. Edited to add

I specifically set the parity for 1 and added the message box so that I can see the order moving it. It always takes the same order, which I say above, but was eager to know whether the reason why it has been chosen is why it chooses the order.

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It seems that all levels continue with an (unrelated) work previously constrained.

The package is stored as an XML .dtsx file. Without hindrances, the objects will be executed in the sequence that appears in the file.