Hi,
Many situations may require a latch state which may remain true preceding the latch instruction, but require to be unlatched while that original latch condition is still true, hence the ONS to only allow the initial latch but not subsequent latches.
One example where I use this is when I require a time to latch a tag.
If I want the turn of a particular hour, for instance, to latch a tag. I use an Equal instruction to compare the PLC Hour with a preset, and a Oneshot to capture the transition to the hour only, and not evaluate the rung true for the entire hour while the Equal instruction remains true. This allows the latched state to be unlatched at any time after the initial latch, even if the logic is still true before the Oneshot in the rung.
Look at what is latching the rung, and if it might require unlatching while that latch state is still true, if so the ONS was necessary.
EDIT: LOL what they said!
G.