A signal checker block serves to check a signal of the control system to be within a given range. As soon as this is longer the case the block will trigger a safety event which can cause the safety system to change its level and handle the situation appropriately. See safetysystem2 for an example.
Please keep in mind that for signals which are of vector or matrix type, each element will be compared to its own upper and lower limit.
When the checker has reached his limit it will fire a safety event exactly once. The safety system then will have to reset the checker by calling

checker.reset();

This will allow a new safety event to be triggered. In case of a matrix type, the checker will fire as soon as at least one of its elements has exceeded its limits.
However, the checker will only fire an event if the safety system and an appropriate event have been registered beforehand by calling

checker.registerSafetyEvent(safetySystem, safetyEvent);

By calling

checker.setActiveLevel(safetyLevel);

the checker will only fire a registered safety event if the current safety level is equal or greater than the safety level set by this method.