The TOF timer DN bit Goes high when the logic to start the timer goes high and turns off when the timer equals the pre-set
The top rung: If A is true and B is false it starts the TON timer, the EN bit is OR(ed) around A so if A goes false the timer is Latched in or the timer will reset.
The TOF timer is initiated by the TON timer , The TOF.DN bit goes high for the time in the accumulator, the NOT TOF.DN in top rung just releases the latch (TOF timer is initiated by a pulse so the DN bit is set high until the timer.ACC value = the pre set).
(assuming A is already off).