Threshold: A boundary: A thing that divides two things from one another. The thing that divides the two things may be as simple as the difference in traits between the two things or a surface.

e.g. a line, a place where one thing becomes another thing such as where the rubber of a tire meets the surface of a road.

Threshold: Boundary between two different things

Boundary: Dividing line, thing that separates two distinct areas

Groupings:

Gradients

Trends

Separate, divide,

Surface: a continuous set of points that has length and breadth but no thickness.
trait: definable attribute
attribute: quality or feature
definable:

Thresholds and Groupings seem to give people a bit of trouble.

Humans like to think simply. So most people tend to think very binary. Actually, binary is an insufficiently correct term here. Binary means “relating to, composed of, or involving two things.” People tend to think in groups. Distinct groups to be more precise. The simplest set of groupings that are distinct from one another in the simplest way is two. Therefore binary is the simplest set of distinct groupings a person could perceive. To see distinctness, there necessarily must be something to compare to. What is compared is the traits.

Differ – traits : different
It’s interesting how “different” is more understood than “differ”.
Differ: not the same
Different: not the same traits

What is the thing that separates water from oil in a container? It isn’t an object or substance. It is the place where the traits between the two things exists as well as illustrates the transition point between the two things.