One Shot vs. FirstScan System Bit

Is there a difference between these two bits?

first scan changes state only on the first scan

a one-shot will change state depending on the rung conditions

I am assuming Allen Bradley.

Yes, there is a difference.

the first scan bit is only executed on the first scan of the program
when the plc is put into run mode or when the power is turned on and the plc executes the first scan of the program.

a one shot can be used any time during the program execution.
it is a single instruction and is true for only one scan cycle.
it looks for a false to true transition.

regards,
james

This may be more than your asking, but:

I would typically use first scan to initialize things on startup of the PLC after power cycle, or whatever may cause it to switch out of run mode.

A one shot I would use for things that I only want to occur one time for every transition, perhaps shifting, or moving data to a new location.