An activity field is a defined portion of the physical universe. Because time is an illusion, an activity field merely defines a sector of space within any given moment. Any and all physical objects are a field of activity—a solar system is no more a field of activity than a molecule.
Below is a diagram showing three different activity fields.
The blue outline represents the field's perimeter.
This is all based upon the principle that spacetime is, in all actuality, space-time-force. The activity in activity fields are the various expressions of force that are embedded in spacetime. Two easily noticed examples of force is the ferromagnetism demonstrated in a horseshoe magnet, and the gravity demonstrated on Earth. The activity being described is what holds stuff together in the first place, which is what allows an individualized thing to be identified as individual.
In theory, the smallest activity field possible for regular matter and energy is a Planck sphere, and can thus be quantized as follows:
A Planck sphere is not a particle, it is a quantization of spacetime travel pertaining to regular, normal matter, predicated upon the values of Planck time and Planck length. It's not a quantization of distance; it's a quantization of the human perception called "traveling in spacetime." The reason it is being described as the quantization of a human perception is because "traveling," as humans perceive it, is in all actuality the teleportation of regular, normal matter and energy.
The teleportation factor of how your physical reality works is fundamental—it is a natural effect of experiencing time. This is because time experience is discontinuous, which is what allows for "change." What you perceive as change from moment to moment is the discontinuity in your time experience's structure. This whole phenomenon of discontinuity in time can be explained by what is called "Planck-state phenomena." A Planck sphere is helping define the human limit to perceiving this multidimensional discontinuity (it's multidimensional because it directly involves space and time). Time is continuously discontinous, and activity fields are portions of the matter and energy that build your continuity experience.