I'm not too sure I understand. I'll try to answer what I THINK you're asking.
There are
Event Responses for just about every
Event.
In a lot of cases we can get a
Point related to our
Event:
-
Events
-

Unit - A unit Begins casting an ability
-

Unit - A unit Begins channeling an ability
-

Unit - A unit Starts the effect of an ability
-
Conditions
-
Actions
-

Set TempPoint = (Target point of ability being cast)
-

Set TempPoint = (Position of (Target unit of ability being cast))
-

Set TempPoint = (Position of (Casting unit))
-
Events
-

Unit - A unit Spawns a summoned unit
-
Conditions
-
Actions
-

Set TempPoint = (Position of (Summoning unit))
-

Set TempPoint = (Position of (Summoned unit))
-
Events
-

Unit - A unit Is issued an order targeting a point
-
Conditions
-
Actions
-

Set TempPoint = (Target point of issued order)
-

Set TempPoint = (Position of (Ordered unit))
-
Events
-

Unit - A unit Is issued an order targeting an object
-
Conditions
-
Actions
-

Set TempPoint = (Position of (Target unit of issued order))
-

Set TempPoint = (Position of (Ordered unit))
But there is no such thing as "entering" a
Point.
Warcraft 3 uses a grid of
X,
Y,
Z coordinates to determine where everything is positioned:
A
Point is an object that stores
X,
Y, and
Z coordinates.
It would be near impossible for you to detect when a unit "enters" those exact coordinates since they're so small and precise. That's why you use
Regions which are rectangles centered at a Point. Regions have a minimum X, maximum X, minimum Y, maximum Y coordinate which can be compared with the X and Y coordinates of a Unit, Item, or Destructible to determine if they're inside of the Region.
With that information you should be able to figure out how to create your own Regions and check if Units are within them. I believe it's easier to do in Jass/Lua (code) since GUI doesn't have all of the tools needed.
But you don't necessarily need to use Warcraft 3's Regions, the concept is easy to recreate.