This is a toggle function that allows you to switch between Z Lock , projecting construction positions to the active CPlane  or not projecting to the active CPlane, (Z Free). You can use the Position Menu to select geometry while forcing or projecting the creation of entities or positions to the currently active CPlane. The default setting is Z Free.

HINT: After changing this function to Z Lock, reset the switch before using other functions (Lower right Status bar will indicate either Z Lock or Z Free).

Z Lock and Z Free Create>Line>Parallel Example: