What are latch (OTL) and unlatch (OTU) instructions in ladder logic?

Latch (OTL - Output Latch) sets a bit ON and keeps it ON even when input condition goes false - retentive output. Unlatch (OTU - Output Unlatch) resets the latched bit to OFF. Latches are used when output must remain in state after momentary input (pushbutton start circuits) or for status flags. Always use both OTL and OTU for the same address in the program; consider fail-safe behavior and power-up state. Standard OTE coils are non-retentive, clearing on power cycle.