The need for a reference model for agent-based modeling and simulation - Modeling and Simulation of Complex Systems - page 69

Multi-Agent- Simulation and Multi-Agent- System are often confused
or at least used synonymously without much care. As time models are
quite a new challenge in the context of Multi-Agent-Systems (cp. [51])
one might easily mix things up.
5.3.4 Agents and objects
A reference model has to support at least two different types of entities:
agents and objects. The term agent refers to an entity with some
kind of autonomous and pro-active behavior (cp. [35, p. 308]). The
term object refers to entities within the agent-based model that do
not exhibit any pro-active behavior, i. e., to entities that are passive
but may, of course, be altered by agents (e.g., items or resources).
Objects are situated within an environment and therefore it is the
choice of a model developer whether a specific part of a model is
represented as object or environment. A good indicator for modeling
parts as an object is the amount of interaction expected with this part.
If the degree of interaction is high, modeling such parts as individual
objects is often beneficial (e.g., picking up items and carrying them
around).
With reference to [35, p. 308], it seems appropriate to point out
that objects in the sense of this reference model are not necessarily
related to objects known from object-oriented programming or other
object-based systems. Although it is of course possible to implement
agents and (model) objects using object-oriented languages, in this
thesis the term object refers to objects as part of the agent-based
model.
5.3.5 Actions, effects and constraints
Actions of agents have a duration and can interfere with each other.
In combination with an explicit time model this imposes especially
that different actions can have different durations. There should be no
need that durations of actions are a multiple of some smallest possible
time step.