Each specific game system object is always based upon a specific compset. These objects are referred to generically as "things" within Hero Lab. For example, the "Longsword" thing might be based upon the "Melee" compset, while the "Fireball" spell would be based on the "Spell" compset.
The compset dictates the general behaviors of all individual things that are based upon it. However, each thing then tailors its own nature and behavior, as appropriate.
All things possess a variety of basic characteristics that are inherently unique to each thing, such as a name and description. Thing also have various characteristics that are inherited from the components from which the thing is derived (which are dictated by the compset). Lastly, things have an assortment of special characteristics that can be optionally specified to customize their individual behavior, including scripts, rules, and a host of other facets (all described elsewhere in this documentation).
Some of you out there in reader-land might be wondering why we used the term "thing" instead of the more standard term "object". The reason is that there are other facets of the underlying HL engine and the Kit that are also very object-oriented in nature. We wanted to be able to use the term "object" in the documentation to refer to general object-oriented behaviors, without specifically linking the word to this one situation. Using the term "thing" for compset-derived objects gives us that ability.