RobotStudio event

Conveyor tracking: eax_f = 0 ? Why?

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Hi everyone!
In my company, some robots use "Conveyor tracking" option, so when modifying targets that are not related to conveyor movement (such as points on stage or in chamber), if i don't run the instruction "ActUnit CNV1;", i have to manually edit value eax_f = 0. But i don't understand clearly why it must be eax_f, can it be eax_e, d,.. or does it depend on some settings that i don't know. And why when i run the instruction "ActUnit CNV1;", i will not have to manually edit value eax_f = 0 anymore!

If someone know the answer, please help me! Thank you all in advance!