It implements all the logic to honor the Trigger List on duty. Its main task is to parse the trigger list, and for each event execute the filter scripts associated with each Level 1 and Level 2 bit that fired for this event. Filter Scripts then call Filters, which call Tools. It then calls Filters and Tools in the appropriate order, returning a trigger decision of Passed or Failed to the Level 3 Framework. The Filters main task is to call Tools to obtain reconstructed physics objects, check these against a set of parameters, and then return a status of Passed or Failed.
Tools are algorithms that reconstruct physics objects.
There can be multiple instances of them distinguishable by their different reference sets of parameters. Most of the available processing time is spent in Tool algorithms and Tools can be called multiple times, so one of their important features is that they cache all of their results. Tools also call other Tools to make their calculations and this is done without any external intervention. Filter Scripts correspond to Level 3 Filtering Trigger Bits and are designed as Logical ANDs of several Filters. They are grouped following each Level 1 and Level 2 Trigger bit, for each bit that passes all associated Level 3 Filters will be tried.